fix(memory): add thread-safe singleton initialization

- Add threading.Lock with double-check locking to ScopeManager
- Add asyncio.Lock with double-check locking to MemoryReflection
- Make reset_memory_metrics async with proper locking
- Update test fixtures to handle async reset functions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/memory/metrics/collector.py

@@ -499,10 +499,11 @@ async def get_memory_metrics() -> MemoryMetrics:
         return _metrics
 
 
-def reset_memory_metrics() -> None:
+async def reset_memory_metrics() -> None:
     """Reset the singleton instance (for testing)."""
     global _metrics
-    _metrics = None
+    async with _lock:
+        _metrics = None
 
 
 # Convenience functions