fix(agents): preserve todos state across node updates (#3180)

* fix(agents): preserve todos state across node updates

ThreadState.todos had no reducer, so any downstream node returning a
partial state without todos was implicitly setting it to None, which
LangGraph then used to overwrite the previously streamed value. This
caused the to-do list to render correctly during streaming but vanish
once streaming completed.

Add a merge_todos reducer that keeps the last non-None value, mirroring
the merge_artifacts pattern already used in the same file. An explicit
empty list is still respected so that 'user cleared todos' works.

Tests: 10 new unit tests in tests/test_thread_state_reducers.py covering
merge_todos plus regression coverage for merge_artifacts and
merge_viewed_images. All 69 thread-related tests pass locally.

Closes #3123

* test(agents): add annotation binding regression guard

Address Copilot review feedback on #3123:

- Add TestThreadStateAnnotations asserting that ThreadState.todos is
  Annotated with merge_todos. Without this guard, reverting the
  Annotated[list | None, merge_todos] binding would silently regress
  #3123 while all existing reducer unit tests continue to pass.

- Align test imports to 'from deerflow.agents.thread_state import ...'
  matching the rest of the backend test suite.

backend/packages/harness/deerflow/agents/thread_state.py

@@ -45,11 +45,24 @@ def merge_viewed_images(existing: dict[str, ViewedImageData] | None, new: dict[s
     return {**existing, **new}
 
 
+def merge_todos(existing: list | None, new: list | None) -> list | None:
+    """Reducer for todos list - keeps the last non-None value.
+
+    Semantics:
+    - If `new` is None (node didn't touch todos), preserve `existing`.
+    - If `new` is provided (even empty list), it represents an explicit
+      update and wins over `existing`.
+    """
+    if new is None:
+        return existing
+    return new
+
+
 class ThreadState(AgentState):
     sandbox: NotRequired[SandboxState | None]
     thread_data: NotRequired[ThreadDataState | None]
     title: NotRequired[str | None]
     artifacts: Annotated[list[str], merge_artifacts]
-    todos: NotRequired[list | None]
+    todos: Annotated[list | None, merge_todos]
     uploaded_files: NotRequired[list[dict] | None]
     viewed_images: Annotated[dict[str, ViewedImageData], merge_viewed_images]  # image_path -> {base64, mime_type}