test(vertex-passthrough): send the virtual key via x-litellm-api-key in streaming tests

The LIT-4761 streaming-classification tests passed only the bring-your-own
Google OAuth token in Authorization and mocked get_litellm_virtual_key, a shape
that cannot authenticate in production. The credential-less filter now resolves
the caller key by auth precedence, so a lone Authorization value reads as the
key and is stripped. Send the virtual key in x-litellm-api-key, matching a real
request, so Authorization is preserved and the classification assertions run.

tests/test_litellm/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py

@@ -3311,6 +3311,7 @@ class TestVertexRawPredictStreamingClassification:
                 "path": f"/vertex_ai/{endpoint}",
                 "headers": [
                     (b"content-type", b"application/json"),
+                    (b"x-litellm-api-key", b"test-key"),
                     (b"authorization", b"Bearer ya29.byo-google-oauth"),
                 ],
                 "query_string": b"",