[电大老baby (HhH666-ux)]

Summary

Every /chat request reads and rewrites the shared agent_state.json / agent_history.json without synchronization. When two requests overlap, their writes can leave a mixture of two JSON documents, and the next request fails with JSONDecodeError: Extra data; the history can also lose updates.

This serializes the read-modify-write cycle behind an asyncio.Lock and writes each document through a temporary file plus os.replace, so a killed or concurrent writer cannot leave a truncated file. The history entries now use model_dump(mode="json"), because the previous model_dump() kept datetimes and json.dumps raised before the write happened.

Tests

• pytest python/samples/agentchat_fastapi/test_app_agent.py -q — 1 passed
• The test posts 10 concurrent /chat requests and asserts every response is 200 and all 20 history entries are present. Without the lock the same test returned 500s and kept only 2 of 20 entries.
• ruff check and ruff format --check (0.4.8, the repo pin) pass on the changed files.

Fixes #8435