fix(types): coerce dict server_tool_use to ServerToolUse in Usage init

Prerequisite for #27346 (completion_cost AttributeError on streaming
Anthropic web_search). #27346's per-chunk coercion is undone by the
existing Usage(**returned_usage.model_dump()) reconstruction in
calculate_usage, which round-trips server_tool_use back to a plain dict;
without this Usage.__init__ coercion the cost path still does attribute
access on a dict and raises. The same prerequisite was bundled into the
stable/1.88.x backport of #27346 (24b9655cd4).

Content-verified present on litellm_internal_staging via aggregator
32c88ca74f (Litellm oss staging 080626, #29932); this restores only the
two-line coercion, not the rest of that aggregator.

(cherry picked from commit 32c88ca74f)

litellm/types/utils.py

@@ -1672,6 +1672,9 @@ class Usage(SafeAttributeModel, CompletionUsage):
             prompt_tokens_details=_prompt_tokens_details or None,
         )
 
+        if isinstance(server_tool_use, dict):
+            server_tool_use = ServerToolUse(**server_tool_use)
+
         if server_tool_use is not None:
             self.server_tool_use = server_tool_use
         else:  # maintain openai compatibility in usage object if possible