fix(storage): harden session append for missing files

[chethanuk]

Summary

• handle raw backend not-found errors during session message appends without crashing
• serialize appends so concurrent first writes do not drop messages
• add focused storage regressions for missing-file and concurrent append behavior

Testing

• pytest tests/storage/test_viking_fs_append.py -v

Notes:

• broader session/server add-message tests are currently blocked here by unrelated existing config/native-binding issues in this environment

Fixes #1358

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PR Reviewer Guide 🔍

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🎫 Ticket compliance analysis 🔶 1358 - Partially compliant Compliant requirements: Fix RuntimeError when appending to missing session files Add regression tests for missing-file and concurrent append behavior Non-compliant requirements: Ensure concurrent appends do not drop messages (due to race condition in fallback lock creation)

⏱️ Estimated effort to review: 3 🔵🔵🔵⚪⚪

🏅 Score: 75

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ Recommended focus areas for review Race Condition in Fallback Lock Creation Concurrent calls to _append_lock_context for the same path may create multiple asyncio.Lock instances because access to _append_fallback_locks is not synchronized. This breaks the serialization guarantee for concurrent appends, potentially leading to lost messages. def _append_lock_context(self, path: str): try: from openviking.storage.transaction import LockContext, get_lock_manager return LockContext(get_lock_manager(), [path], lock_mode="point") except RuntimeError: lock = self._append_fallback_locks.get(path) if lock is None: lock = asyncio.Lock() self._append_fallback_locks[path] = lock return _asyncio_lock_context(lock) Unused Dead Code The _noop_async_lock async context manager is defined but never used. @asynccontextmanager async def _noop_async_lock() -> Any: yield

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