fix: validate response has request before using in BadRequestError

Previously, BadRequestError would use any passed httpx.Response object
directly. However, httpx.Response objects require a request attribute
to be set. When a response without a request was passed (e.g., in tests),
it would cause a RuntimeError: 'The request instance has not been set
on this response.'

Now we validate that the response has a valid request attribute before
using it. If not, we fall back to the cached minimal error response,
matching the behavior before the optimization commit 4e3e1d54f5.

Fixes test_context_window_exceeded_error_from_litellm_proxy test failure.

litellm/exceptions.py

@@ -142,7 +142,12 @@ class BadRequestError(openai.BadRequestError):  # type: ignore
         self.litellm_debug_info = litellm_debug_info
         self.max_retries = max_retries
         self.num_retries = num_retries
-        if response is not None and isinstance(response, httpx.Response):
+        if (
+            response is not None
+            and isinstance(response, httpx.Response)
+            and hasattr(response, "request")
+            and response.request is not None
+        ):
             self.response = response
         else:
             self.response = _get_minimal_error_response()