refactor(gateway): route all thread metadata access through ThreadMetaStore

Following the rename/delete bug fix in PR1, migrate the remaining direct
LangGraph Store reads/writes in the threads router and services to the
ThreadMetaStore abstraction so that the sqlite and memory backends behave
identically and the legacy dual-write paths can be removed.

Migrated endpoints (threads.py):
- create_thread: idempotency check + write now use thread_meta_repo.get/create
  instead of dual-writing the LangGraph Store and the SQL row.
- get_thread: reads from thread_meta_repo.get; the checkpoint-only fallback
  for legacy threads is preserved.
- patch_thread: replaced _store_get/_store_put with thread_meta_repo.update_metadata.
- delete_thread_data: dropped the legacy store.adelete; thread_meta_repo.delete
  already covers it.

Removed dead code (services.py):
- _upsert_thread_in_store — redundant with the immediately following
  thread_meta_repo.create() call.
- _sync_thread_title_after_run — worker.py's finally block already syncs
  the title via thread_meta_repo.update_display_name() after each run.

Removed dead code (threads.py):
- _store_get / _store_put / _store_upsert helpers (no remaining callers).
- THREADS_NS constant.
- get_store import (router no longer touches the LangGraph Store directly).

New abstract method:
- ThreadMetaStore.update_metadata(thread_id, metadata) merges metadata into
  the thread's metadata field. Implemented in both ThreadMetaRepository (SQL,
  read-modify-write inside one session) and MemoryThreadMetaStore. Three new
  unit tests cover merge / empty / nonexistent behaviour.

Net change: -134 lines. Full test suite: 1693 passed, 14 skipped.
Verified end-to-end with curl in gateway mode against sqlite backend
(create / patch / get / rename / search / delete).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/gateway/services.py

@@ -12,7 +12,6 @@ import dataclasses
 import json
 import logging
 import re
-import time
 from typing import Any
 
 from fastapi import HTTPException, Request
@@ -173,71 +172,6 @@ def build_run_config(
 # ---------------------------------------------------------------------------
 
 
-async def _upsert_thread_in_store(store, thread_id: str, metadata: dict | None) -> None:
-    """Create or refresh the thread record in the Store.
-
-    Called from :func:`start_run` so that threads created via the stateless
-    ``/runs/stream`` endpoint (which never calls ``POST /threads``) still
-    appear in ``/threads/search`` results.
-    """
-    # Deferred import to avoid circular import with the threads router module.
-    from app.gateway.routers.threads import _store_upsert
-
-    try:
-        await _store_upsert(store, thread_id, metadata=metadata)
-    except Exception:
-        logger.warning("Failed to upsert thread %s in store (non-fatal)", sanitize_log_param(thread_id))
-
-
-async def _sync_thread_title_after_run(
-    run_task: asyncio.Task,
-    thread_id: str,
-    checkpointer: Any,
-    store: Any,
-) -> None:
-    """Wait for *run_task* to finish, then persist the generated title to the Store.
-
-    TitleMiddleware writes the generated title to the LangGraph agent state
-    (checkpointer) but the Gateway's Store record is not updated automatically.
-    This coroutine closes that gap by reading the final checkpoint after the
-    run completes and syncing ``values.title`` into the Store record so that
-    subsequent ``/threads/search`` responses include the correct title.
-
-    Runs as a fire-and-forget :func:`asyncio.create_task`; failures are
-    logged at DEBUG level and never propagate.
-    """
-    # Wait for the background run task to complete (any outcome).
-    # asyncio.wait does not propagate task exceptions — it just returns
-    # when the task is done, cancelled, or failed.
-    await asyncio.wait({run_task})
-
-    # Deferred import to avoid circular import with the threads router module.
-    from app.gateway.routers.threads import _store_get, _store_put
-
-    try:
-        ckpt_config = {"configurable": {"thread_id": thread_id, "checkpoint_ns": ""}}
-        ckpt_tuple = await checkpointer.aget_tuple(ckpt_config)
-        if ckpt_tuple is None:
-            return
-
-        channel_values = ckpt_tuple.checkpoint.get("channel_values", {})
-        title = channel_values.get("title")
-        if not title:
-            return
-
-        existing = await _store_get(store, thread_id)
-        if existing is None:
-            return
-
-        updated = dict(existing)
-        updated.setdefault("values", {})["title"] = title
-        updated["updated_at"] = time.time()
-        await _store_put(store, updated)
-        logger.debug("Synced title %r for thread %s", title, thread_id)
-    except Exception:
-        logger.debug("Failed to sync title for thread %s (non-fatal)", thread_id, exc_info=True)
-
-
 async def start_run(
     body: Any,
     thread_id: str,
@@ -291,12 +225,9 @@ async def start_run(
     except UnsupportedStrategyError as exc:
         raise HTTPException(status_code=501, detail=str(exc)) from exc
 
-    # Ensure the thread is visible in /threads/search, even for threads that
-    # were never explicitly created via POST /threads (e.g. stateless runs).
-    if run_ctx.store is not None:
-        await _upsert_thread_in_store(run_ctx.store, thread_id, body.metadata)
-
-    # Upsert thread metadata so the thread appears in /threads/search
+    # Upsert thread metadata so the thread appears in /threads/search,
+    # even for threads that were never explicitly created via POST /threads
+    # (e.g. stateless runs).
     try:
         existing = await run_ctx.thread_meta_repo.get(thread_id)
         if existing is None:
@@ -353,11 +284,9 @@ async def start_run(
     )
     record.task = task
 
-    # After the run completes, sync the title generated by TitleMiddleware from
-    # the checkpointer into the Store record so that /threads/search returns the
-    # correct title instead of an empty values dict.
-    if run_ctx.store is not None:
-        asyncio.create_task(_sync_thread_title_after_run(task, thread_id, run_ctx.checkpointer, run_ctx.store))
+    # Title sync is handled by worker.py's finally block which reads the
+    # title from the checkpoint and calls thread_meta_repo.update_display_name
+    # after the run completes.
 
     return record