fix(types): fix mypy errors in pass-through endpoint query param types

- passthrough/utils.py: change `request_query_params` from `Dict` to
  `Mapping` so callers passing `dict[str, str]` satisfy the type checker
  (dict is invariant; Mapping is covariant in its value type)
- pass_through_endpoints.py: cast `param_default_query_params` to
  `Optional[dict]` consistent with the existing cast pattern for other
  params extracted from the untyped `target_params` dict

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/passthrough/utils.py

@@ -1,4 +1,4 @@
-from typing import Dict, List, Optional, Union
+from typing import Dict, List, Mapping, Optional, Union
 from urllib.parse import parse_qs
 
 import httpx
@@ -10,7 +10,7 @@ class BasePassthroughUtils:
     @staticmethod
     def get_merged_query_parameters(
         existing_url: httpx.URL,
-        request_query_params: Dict[str, Union[str, list]],
+        request_query_params: Mapping[str, Union[str, list]],
         default_query_params: Optional[Dict[str, Union[str, list]]] = None
     ) -> Dict[str, Union[str, List[str]]]:
         # Get the existing query params from the target URL

litellm/proxy/pass_through_endpoints/pass_through_endpoints.py

@@ -1221,7 +1221,7 @@ def create_pass_through_route(
                 forward_headers=cast(Optional[bool], param_forward_headers),
                 merge_query_params=cast(Optional[bool], param_merge_query_params),
                 query_params=final_query_params,
-                default_query_params=param_default_query_params,
+                default_query_params=cast(Optional[dict], param_default_query_params),
                 stream=is_streaming_request or stream,
                 custom_body=final_custom_body,
                 cost_per_request=cast(Optional[float], param_cost_per_request),