fix(memory): prevent entry metadata mutation in vector search

- Create shallow copy of VectorIndexEntry when adding similarity score
- Prevents mutation of cached entries that could corrupt shared state

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

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

backend/app/services/memory/indexing/index.py

@@ -197,10 +197,17 @@ class VectorIndex(MemoryIndex[T]):
             results = [(s, e) for s, e in results if e.memory_type == memory_type]
 
         # Store similarity in metadata for the returned entries
+        # Use a copy of metadata to avoid mutating cached entries
         output = []
         for similarity, entry in results[:limit]:
-            entry.metadata["similarity"] = similarity
-            output.append(entry)
+            # Create a shallow copy of the entry with updated metadata
+            entry_with_score = VectorIndexEntry(
+                memory_id=entry.memory_id,
+                memory_type=entry.memory_type,
+                embedding=entry.embedding,
+                metadata={**entry.metadata, "similarity": similarity},
+            )
+            output.append(entry_with_score)
 
         logger.debug(f"Vector search returned {len(output)} results")
         return output