* Fix remaining VCR live-call leaks
* test(vcr): dedupe live-test helpers and drop spurious kwargs
Extract the duplicated isVertexQuotaError/runVertexRequestOrSkip Vertex
quota-skip helpers into tests/pass_through_tests/vertex_test_helpers.js and the
duplicated _skip_live_prompt_caching_test guard into tests/_live_test_helpers.py
so each lives in one place. In test_aarun_thread_litellm, build a separate
message_data carrying role/content for add_message and a thread_data without
them for run_thread/run_thread_stream/get_messages, which no longer receive the
spurious message fields.
* test(overhead): assert mock transport is exercised in non-streaming and stream tests
