* 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.
Huixin615