fix(agents): preserve todos state across node updates

[Huixin615]

What

Adds a merge_todos reducer to ThreadState.todos so that a downstream node returning a partial state without todos no longer wipes the previously streamed value.

Why

Closes #3123.

ThreadState.todos was declared as NotRequired[list | None] with no reducer. In LangGraph, fields without a reducer use last-write-wins, and a missing key in a partial state update is materialized as None, which overwrites the previous value. This is exactly the root cause @WillemJiang identified in the issue: the to-do list renders correctly during streaming, then disappears once a later node finishes with no todos in its return.

How

• Add merge_todos(existing, new) reducer in backend/packages/harness/deerflow/agents/thread_state.py. Semantics:
  • new is None → preserve existing (the actual fix for To-dos 列表在流式输出过程中显示，但输出完成后消失 #3123)
  • new is not None (including []) → new wins, so explicit clears still work
• Annotate todos: Annotated[list | None, merge_todos], mirroring the existing artifacts: Annotated[list[str], merge_artifacts] pattern in the same file.

Tests

New backend/tests/test_thread_state_reducers.py with 10 cases:

• merge_todos (5 cases, including the regression test for To-dos 列表在流式输出过程中显示，但输出完成后消失 #3123):
  • new value overrides existing
  • new=None preserves existing ← regression coverage for To-dos 列表在流式输出过程中显示，但输出完成后消失 #3123
  • existing=None accepts new
  • both None returns None
  • empty list [] is treated as explicit clear
• Sanity checks for the existing reducers merge_artifacts (3 cases) and merge_viewed_images (2 cases) to lock in current behaviour.

All 69 thread-related tests pass locally:

PYTHONPATH=. uv run pytest \
  tests/test_thread_state_reducers.py \
  tests/test_thread_meta_repo.py \
  tests/test_memory_thread_meta_isolation.py \
  tests/test_thread_data_middleware.py -v
# 69 passed in 1.96s

make lint and make format both pass cleanly (only the 2 files in this PR are touched).

Checklist

• make lint passes (ruff check + ruff format --check)
• make format applied
• New unit tests added and passing
• No public API changes
• Scoped change: only 2 files touched

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

Adds a reducer for ThreadState.todos so LangGraph partial state updates that omit todos do not clear previously streamed todo state, addressing issue #3123.

Changes:

• Adds merge_todos(existing, new) with “preserve on None, replace otherwise” semantics.
• Annotates ThreadState.todos with the new reducer.
• Adds reducer unit tests for todos plus sanity checks for existing artifact/image reducers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
backend/packages/harness/deerflow/agents/thread_state.py	Defines and wires the merge_todos reducer into ThreadState.todos.
backend/tests/test_thread_state_reducers.py	Adds unit coverage for reducer behavior and existing reducer sanity checks.

[WillemJiang]

@Huixin615, thanks for your contribution. Please take a look at the review comment of Copilot.