fix(server): handle CancelledError during shutdown paths

[ZaynJarvis]

previous issue is Exception doesn't catch asyncio.CancelledError. so when asyncio.CancelledError happens, server crash.

import asyncio

print("MRO:", asyncio.CancelledError.__mro__)
print("issubclass(asyncio.CancelledError, Exception):", issubclass(asyncio.CancelledError, Exception))

try:
    raise asyncio.CancelledError("demo")
except Exception:
    print("caught by Exception")
except BaseException as e:
    print("not caught by Exception; caught by BaseException:", type(e).__name__, str(e))

===
MRO: (<class 'asyncio.exceptions.CancelledError'>, <class 'BaseException'>, <class 'object'>)
issubclass(asyncio.CancelledError, Exception): False
not caught by Exception; caught by BaseException: CancelledError demo

Summary

• handle asyncio.CancelledError explicitly in shutdown-sensitive server paths
• only downgrade dedup cancellation to a fallback when vector storage is actually closing
• add regression tests for background commit cancellation, lifespan shutdown, and dedup/redo cancellation semantics

Testing

• python3 -m ruff check openviking/session/memory_deduplicator.py openviking/storage/transaction/lock_manager.py openviking/server/app.py openviking/server/routers/sessions.py tests/session/test_memory_dedup_actions.py tests/transaction/test_lock_manager.py tests/test_session_task_tracking.py tests/server/test_server_health.py
• python3 -m pytest tests/session/test_memory_dedup_actions.py tests/transaction/test_lock_manager.py tests/test_session_task_tracking.py tests/server/test_server_health.py

[github-actions]

Failed to generate code suggestions for PR