🐛 Fix duplicate operationId when using multiple methods on same route

When a route is added with multiple HTTP methods (e.g., methods=["POST", "DELETE"]),
all methods were getting the same auto-generated operationId based only on the
first method, causing "Duplicate Operation ID" warnings.

The fix appends the method name to the operationId when:
1. No explicit operation_id is provided
2. The route has multiple methods

This ensures unique operationIds for each method while maintaining backward
compatibility for single-method routes and explicit operation_id usage.

Fixes #13175

fastapi/openapi/utils.py

@@ -243,6 +243,10 @@ def get_openapi_operation_metadata(
     if route.description:
         operation["description"] = route.description
     operation_id = route.operation_id or route.unique_id
+    # When a route has multiple methods and uses auto-generated unique_id,
+    # append the method to make the operation_id unique for each method
+    if not route.operation_id and len(route.methods) > 1:
+        operation_id = f"{operation_id}_{method.lower()}"
     if operation_id in operation_ids:
         message = (
             f"Duplicate Operation ID {operation_id} for function "