Add return type hint to forward_headers_from_request method (#23693)

- Added missing Dict return type annotation to improve type safety
- Enhances IDE autocomplete and static type checking
- No functional changes, purely type annotation improvement

Co-authored-by: Vivek Vardhan <vivekvar.dl@gmail.com>

litellm/passthrough/utils.py

@@ -40,7 +40,7 @@ class BasePassthroughUtils:
         request_headers: dict,
         headers: dict,
         forward_headers: Optional[bool] = False,
-    ):
+    ) -> dict:
         """
         Helper to forward headers from original request.