feat(run): Propagates model_name from the gateway request through the runtime and persistence stack to the SQLite database. (#2775)

* feat(run): propagate model_name from gateway request context to persistence layer

Pass model_name through the full run creation pipeline — from
RunCreateRequest.context in the gateway, through RunManager, to the
RunStore interface and SQL persistence. This enables client-specified
model selection to be recorded per-run in the database.

* feat(run): add model allowlist validation and effective model name capture

- Validate model_name against allowlist in gateway services.py using
  get_app_config().get_model_config()
- Truncate model_name to 128 chars to match DB column constraint
- In worker.py, capture effective model name from agent.metadata after
  agent creation and persist if resolved differently than requested

* feat(run): add defense-in-depth model_name normalization and round-trip persistence tests

- Add _normalize_model_name() to RunRepository for whitespace stripping
  and 128-char truncation before DB writes.
- Add round-trip unit tests for model_name creation and default None
  in test_run_manager.py.

* fix(run): coerce non-string model_name values before strip/truncate in _normalize_model_name

* fix(gateway): add runtime type guard for model_name coercion in gateway services

Add isinstance check and str() coercion before calling .strip() to prevent
AttributeError when non-string types (int, None, etc.) flow through the
gateway. Paired with SQL integration test for end-to-end model_name
persistence across gateway → langgraph → persistence layer.

* fix(run): drop Alembic migration for model_name (no-op) and expose public update method on RunManager

- Drop a1b2c3d4e5f6 migration: model_name already exists in RunRow schema
  and is auto-created via Base.metadata.create_all() at startup
- Add update_model_name() public method to RunManager to replace the private
  _persist_to_store call in worker.py, preserving internal locking/persistence

backend/tests/test_run_repository.py

@@ -249,3 +249,32 @@ class TestRunRepository:
         rows = await repo.list_by_thread("t1", user_id=None)
         assert len(rows) == 2
         await _cleanup()
+
+    @pytest.mark.anyio
+    async def test_model_name_persistence(self, tmp_path):
+        """RunRepository should persist, normalize, and truncate model_name correctly via SQL."""
+        from deerflow.persistence.engine import get_session_factory, init_engine
+
+        url = f"sqlite+aiosqlite:///{tmp_path / 'test.db'}"
+        await init_engine("sqlite", url=url, sqlite_dir=str(tmp_path))
+        repo = RunRepository(get_session_factory())
+
+        await repo.put("run-1", thread_id="thread-1", model_name="gpt-4o")
+        row = await repo.get("run-1")
+        assert row is not None
+        assert row["model_name"] == "gpt-4o"
+
+        long_name = "a" * 200
+        await repo.put("run-2", thread_id="thread-1", model_name=long_name)
+        row2 = await repo.get("run-2")
+        assert row2["model_name"] == "a" * 128
+
+        await repo.put("run-3", thread_id="thread-1", model_name=123)
+        row3 = await repo.get("run-3")
+        assert row3["model_name"] == "123"
+
+        await repo.put("run-4", thread_id="thread-1", model_name=None)
+        row4 = await repo.get("run-4")
+        assert row4["model_name"] is None
+
+        await _cleanup()