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/packages/harness/deerflow/runtime/runs/manager.py

@@ -6,9 +6,10 @@ import asyncio
 import logging
 import uuid
 from dataclasses import dataclass, field
-from datetime import UTC, datetime
 from typing import TYPE_CHECKING
 
+from deerflow.utils.time import now_iso as _now_iso
+
 from .schemas import DisconnectMode, RunStatus
 
 if TYPE_CHECKING:
@@ -17,10 +18,6 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-def _now_iso() -> str:
-    return datetime.now(UTC).isoformat()
-
-
 @dataclass
 class RunRecord:
     """Mutable record for a single run."""