perf: skip ThreadPoolExecutor when graph store is disabled

Avoids spawning 2 threads per add() call when graph is not configured.
Fixes thread leakage under high concurrency (was accumulating 3500+ threads
per worker). Calls _add_to_vector_store directly in the current thread.

Applied to both sync Memory and async AsyncMemory classes.

mem0/memory/main.py

@@ -454,6 +454,7 @@ class Memory(MemoryBase):
         else:
             messages = parse_vision_messages(messages)
 
+        if self.enable_graph:
             with concurrent.futures.ThreadPoolExecutor() as executor:
                 future1 = executor.submit(self._add_to_vector_store, messages, processed_metadata, effective_filters, infer, timestamp)
                 future2 = executor.submit(self._add_to_graph, messages, effective_filters)
@@ -463,12 +464,12 @@ class Memory(MemoryBase):
                 vector_store_result = future1.result()
                 graph_result = future2.result()
 
-        if self.enable_graph:
             return {
                 "results": vector_store_result,
                 "relations": graph_result,
             }
 
+        vector_store_result = self._add_to_vector_store(messages, processed_metadata, effective_filters, infer, timestamp)
         return {"results": vector_store_result}
 
     def _add_to_vector_store(self, messages, metadata, filters, infer, timestamp=None):
@@ -1737,6 +1738,7 @@ class AsyncMemory(MemoryBase):
         else:
             messages = parse_vision_messages(messages)
 
+        if self.enable_graph:
             vector_store_task = asyncio.create_task(
                 self._add_to_vector_store(messages, processed_metadata, effective_filters, infer)
             )
@@ -1744,12 +1746,12 @@ class AsyncMemory(MemoryBase):
 
             vector_store_result, graph_result = await asyncio.gather(vector_store_task, graph_task)
 
-        if self.enable_graph:
             return {
                 "results": vector_store_result,
                 "relations": graph_result,
             }
 
+        vector_store_result = await self._add_to_vector_store(messages, processed_metadata, effective_filters, infer)
         return {"results": vector_store_result}
 
     async def _add_to_vector_store(