feat: OpenAIVectorIOMixin for vector_stores common logic (#2427)

Extracts common OpenAI vector-store code into its own mixin so that all
providers can share the same core logic.
This also makes it easy for Llama Stack to support both vector-stores
and Llama Stack APIs in the interim so that both share the same
underlying vector-dbs.

Each provider contains storage specific logic to `create / edit / delete
/ list` vector dbs while the plumbing logic is standardized in the
common code.

Ensured that this works well with both faiss and sqllite-vec. 

### Test Plan 
```
llama stack run starter
pytest -sv --stack-config http://localhost:8321 tests/integration/vector_io/test_openai_vector_stores.py --embedding-model all-MiniLM-L6-v2
```

llama_stack/distribution/resolver.py

@@ -394,9 +394,13 @@ def check_protocol_compliance(obj: Any, protocol: Any) -> None:
                     logger.error(f"Method {name} incompatible proto: {proto_params} vs. obj: {obj_params}")
                     missing_methods.append((name, "signature_mismatch"))
                 else:
-                    # Check if the method is actually implemented in the class
-                    method_owner = next((cls for cls in mro if name in cls.__dict__), None)
-                    if method_owner is None or method_owner.__name__ == protocol.__name__:
+                    # Check if the method has a concrete implementation (not just a protocol stub)
+                    # Find all classes in MRO that define this method
+                    method_owners = [cls for cls in mro if name in cls.__dict__]
+
+                    # Allow methods from mixins/parents, only reject if ONLY the protocol defines it
+                    if len(method_owners) == 1 and method_owners[0].__name__ == protocol.__name__:
+                        # Only reject if the method is ONLY defined in the protocol itself (abstract stub)
                         missing_methods.append((name, "not_actually_implemented"))
 
     if missing_methods: