fix(otel): annotate to_openai_dict as Mapping after the rebase onto main

Main's type-discipline budget tightened since the branch point; the plain
dict return annotation is the one violation the rebased branch adds.
Callers only serialize the result, so the read-only view is accurate.

litellm/integrations/otel/mappers/utils.py

@@ -21,7 +21,12 @@ class MessageToolCall:
     name: str | None
     arguments: str | None
 
-    def to_openai_dict(self) -> dict[str, object]:
+    def to_openai_dict(self) -> Mapping[str, object]:
+        """The OpenAI wire shape of this tool call.
+
+        The runtime object is a plain dict (it is only ever JSON-serialized);
+        the ``Mapping`` return type marks it read-only to callers.
+        """
         return {
             "id": self.id,
             "type": self.type,