fix(gateway): return ISO 8601 timestamps from threads endpoints (#2599)

* fix(gateway): return ISO 8601 timestamps from threads endpoints (#2594)

ThreadResponse documents created_at / updated_at as ISO timestamps,
matching the LangGraph Platform schema (langgraph_sdk.schema.Thread
exposes them as datetime, JSON-encoded as ISO 8601). The gateway
threads router was instead emitting str(time.time()) — unix-second
floats — breaking frontend new Date() parsing and producing a mixed
ISO/unix wire format that also corrupted the search sort order.

Centralize timestamp generation in deerflow.utils.time:
- now_iso()       — datetime.now(UTC).isoformat()
- coerce_iso(x)   — heals legacy unix-timestamp strings on read so the
                    store converges to ISO without a one-shot migration

threads.py: replace 6 time.time() call sites with now_iso(); wrap all
read paths and Phase-2 checkpoint metadata with coerce_iso(); _store_upsert
opportunistically heals legacy created_at on update; drop unused time import.

thread_runs.py: reuse now_iso() instead of a private duplicate _now_iso(),
preventing future drift between the two timestamp call sites.

Tests: 9 unit tests for the helper; 5 integration tests pinning the ISO
contract for create/get/patch/search and the legacy-healing path on the
internal store upsert. Full suite: 2144 passed, 15 skipped, 0 failed.

Closes #2594

* fix(gateway): coerce checkpoint metadata timestamps to ISO on read

After the merge with main, three additional read paths in ``threads.py``
were still emitting raw ``str(metadata.get("created_at", ""))`` —
``get_thread_state``, ``update_thread_state``, and ``get_thread_history``.

Same root cause as #2594: when the checkpoint metadata's ``created_at``
is a unix-second float (legacy data, or a checkpoint written by an older
Gateway version), ``str(float)`` produces ``"1777252410.411327"`` and the
frontend's ``new Date(...)`` returns ``Invalid Date``. The fix on the
``/threads/{id}`` GET path was already in place; these three sibling
endpoints needed the same treatment.

All four call sites now flow through ``coerce_iso``, so:
- legacy float metadata heals to ISO on the way out,
- ISO metadata passes through unchanged,
- ``datetime`` instances (which the new ``coerce_iso`` branch handles
  explicitly) emit with the ``T`` separator instead of falling through
  to the space-separated ``str(datetime)`` form.

Coverage added for the two endpoints not already pinned by the merge:
- ``test_get_thread_state_returns_iso_for_legacy_checkpoint_metadata``
- ``test_get_thread_history_returns_iso_for_legacy_checkpoint_metadata``

Both pre-seed a checkpoint whose metadata carries the literal float
from the issue body and assert the wire format is ISO.

backend/app/gateway/routers/threads.py

@@ -13,7 +13,6 @@ matching the LangGraph Platform wire format expected by the
 from __future__ import annotations
 
 import logging
-import time
 import uuid
 from typing import Any
 
@@ -27,6 +26,7 @@ from app.gateway.utils import sanitize_log_param
 from deerflow.config.paths import Paths, get_paths
 from deerflow.runtime import serialize_channel_values
 from deerflow.runtime.user_context import get_effective_user_id
+from deerflow.utils.time import coerce_iso, now_iso
 
 logger = logging.getLogger(__name__)
 router = APIRouter(prefix="/api/threads", tags=["threads"])
@@ -234,7 +234,7 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
     checkpointer = get_checkpointer(request)
     thread_store = get_thread_store(request)
     thread_id = body.thread_id or str(uuid.uuid4())
-    now = time.time()
+    now = now_iso()
     # ``body.metadata`` is already stripped of server-reserved keys by
     # ``ThreadCreateRequest._strip_reserved`` — see the model definition.
 
@@ -244,8 +244,8 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
         return ThreadResponse(
             thread_id=thread_id,
             status=existing_record.get("status", "idle"),
-            created_at=str(existing_record.get("created_at", "")),
-            updated_at=str(existing_record.get("updated_at", "")),
+            created_at=coerce_iso(existing_record.get("created_at", "")),
+            updated_at=coerce_iso(existing_record.get("updated_at", "")),
             metadata=existing_record.get("metadata", {}),
         )
 
@@ -280,8 +280,8 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
     return ThreadResponse(
         thread_id=thread_id,
         status="idle",
-        created_at=str(now),
-        updated_at=str(now),
+        created_at=now,
+        updated_at=now,
         metadata=body.metadata,
     )
 
@@ -306,8 +306,11 @@ async def search_threads(body: ThreadSearchRequest, request: Request) -> list[Th
         ThreadResponse(
             thread_id=r["thread_id"],
             status=r.get("status", "idle"),
-            created_at=r.get("created_at", ""),
-            updated_at=r.get("updated_at", ""),
+            # ``coerce_iso`` heals legacy unix-second values that
+            # ``MemoryThreadMetaStore`` historically wrote with ``time.time()``;
+            # SQL-backed rows already arrive as ISO strings and pass through.
+            created_at=coerce_iso(r.get("created_at", "")),
+            updated_at=coerce_iso(r.get("updated_at", "")),
             metadata=r.get("metadata", {}),
             values={"title": r["display_name"]} if r.get("display_name") else {},
             interrupts={},
@@ -339,8 +342,8 @@ async def patch_thread(thread_id: str, body: ThreadPatchRequest, request: Reques
     return ThreadResponse(
         thread_id=thread_id,
         status=record.get("status", "idle"),
-        created_at=str(record.get("created_at", "")),
-        updated_at=str(record.get("updated_at", "")),
+        created_at=coerce_iso(record.get("created_at", "")),
+        updated_at=coerce_iso(record.get("updated_at", "")),
         metadata=record.get("metadata", {}),
     )
 
@@ -380,8 +383,8 @@ async def get_thread(thread_id: str, request: Request) -> ThreadResponse:
         record = {
             "thread_id": thread_id,
             "status": "idle",
-            "created_at": ckpt_meta.get("created_at", ""),
-            "updated_at": ckpt_meta.get("updated_at", ckpt_meta.get("created_at", "")),
+            "created_at": coerce_iso(ckpt_meta.get("created_at", "")),
+            "updated_at": coerce_iso(ckpt_meta.get("updated_at", ckpt_meta.get("created_at", ""))),
             "metadata": {k: v for k, v in ckpt_meta.items() if k not in ("created_at", "updated_at", "step", "source", "writes", "parents")},
         }
 
@@ -395,8 +398,8 @@ async def get_thread(thread_id: str, request: Request) -> ThreadResponse:
     return ThreadResponse(
         thread_id=thread_id,
         status=status,
-        created_at=str(record.get("created_at", "")),
-        updated_at=str(record.get("updated_at", "")),
+        created_at=coerce_iso(record.get("created_at", "")),
+        updated_at=coerce_iso(record.get("updated_at", "")),
         metadata=record.get("metadata", {}),
         values=serialize_channel_values(channel_values),
     )
@@ -447,10 +450,10 @@ async def get_thread_state(thread_id: str, request: Request) -> ThreadStateRespo
         values=values,
         next=next_tasks,
         metadata=metadata,
-        checkpoint={"id": checkpoint_id, "ts": str(metadata.get("created_at", ""))},
+        checkpoint={"id": checkpoint_id, "ts": coerce_iso(metadata.get("created_at", ""))},
         checkpoint_id=checkpoint_id,
         parent_checkpoint_id=parent_checkpoint_id,
-        created_at=str(metadata.get("created_at", "")),
+        created_at=coerce_iso(metadata.get("created_at", "")),
         tasks=tasks,
     )
 
@@ -500,7 +503,7 @@ async def update_thread_state(thread_id: str, body: ThreadStateUpdateRequest, re
         channel_values.update(body.values)
 
     checkpoint["channel_values"] = channel_values
-    metadata["updated_at"] = time.time()
+    metadata["updated_at"] = now_iso()
 
     if body.as_node:
         metadata["source"] = "update"
@@ -541,7 +544,7 @@ async def update_thread_state(thread_id: str, body: ThreadStateUpdateRequest, re
         next=[],
         metadata=metadata,
         checkpoint_id=new_checkpoint_id,
-        created_at=str(metadata.get("created_at", "")),
+        created_at=coerce_iso(metadata.get("created_at", "")),
     )
 
 
@@ -608,7 +611,7 @@ async def get_thread_history(thread_id: str, body: ThreadHistoryRequest, request
                     parent_checkpoint_id=parent_id,
                     metadata=user_meta,
                     values=values,
-                    created_at=str(metadata.get("created_at", "")),
+                    created_at=coerce_iso(metadata.get("created_at", "")),
                     next=next_tasks,
                 )
             )