fix(backend): remove invalid avatar access in task sync

[FicoHu]

Summary

• stop incremental WebSocket task sync from reading the nonexistent User.avatar ORM attribute
• align incremental sender payloads with the existing live-message payload shape
• add regression coverage for user-authored incremental history entries

Production evidence

The hourly production patrol found 8 task:join failures with the stable signature 'User' object has no attribute 'avatar'. The shared User ORM model defines id and user_name but no avatar column; the live WebSocket sender payload already sends only those two fields.

Tests

• cd backend && uv run pytest tests/api/ws/test_chat_namespace_fork_history.py -q — 3 passed
• cd backend && uv run black --check app/api/ws/chat_namespace.py tests/api/ws/test_chat_namespace_fork_history.py

Summary by CodeRabbit

• Bug Fixes
  • Updated chat subtask data to provide user sender details without including avatar information.
  • Preserved sender information for user-generated subtasks while keeping non-user subtasks without a sender.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: d676e2a3-9e03-4701-8626-24f149a251a2

📥 Commits

Reviewing files that changed from the base of the PR and between 73cf6e2 and d777443.

📒 Files selected for processing (2)

• backend/app/api/ws/chat_namespace.py
• backend/tests/api/ws/test_chat_namespace_fork_history.py

💤 Files with no reviewable changes (1)

• backend/app/api/ws/chat_namespace.py

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📝 Walkthrough

Walkthrough

The incremental task-join serialization removes avatar from user sender data while retaining user identifiers and adds fork-history test coverage.

Changes

Incremental sender serialization

Layer / File(s)	Summary
Update sender payload and validation backend/app/api/ws/chat_namespace.py, backend/tests/api/ws/test_chat_namespace_fork_history.py	User sender data now includes user_id and user_name without avatar; a test verifies serialization for a fork-history user subtask.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main fix: removing invalid avatar access during backend task synchronization.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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