refactor: rename local variable extensions to extensions_to_use for clarity

Author: guglielmo-san

src/a2a/client/transports/grpc.py

@@ -64,9 +64,9 @@ def _get_grpc_metadata(
         extensions: list[str] | None = None,
     ) -> list[tuple[str, str]] | None:
         """Creates gRPC metadata for extensions."""
-        extensions = extensions if extensions is not None else self.extensions
-        if extensions is not None:
-            return [(HTTP_EXTENSION_HEADER.lower(), ','.join(extensions))]
+        extensions_to_use = extensions if extensions is not None else self.extensions
+        if extensions_to_use is not None:
+            return [(HTTP_EXTENSION_HEADER.lower(), ','.join(extensions_to_use))]
         return None
 
     @classmethod