* test(pass-through): move Gemini pass-through tests to gemini-3.1-flash-lite
gemini-2.5-flash-lite is a generation behind and is slated for discontinuation on Vertex AI no earlier than October 16, 2026, so the pass-through suite was exercising an aging model. Every reference now points at gemini-3.1-flash-lite, which is GA and already priced in the cost map so the spend-logging assertions still compute a real cost
test_vertex.test.js also gains jest.retryTimes(3) to match the sibling spend tests. The CI failures were intermittent 429 RESOURCE_EXHAUSTED from Vertex quota pressure, and that file was the only one without a retry, so a single rate-limited request was failing the whole job
* test(pass-through): point Vertex tests at the global endpoint for gemini-3.1-flash-lite
gemini-3.1-flash-lite is not served on the Vertex us-central1 regional endpoint for the CI project, so the Vertex pass-through tests were returning a deterministic 404 "Publisher Model ... was not found or your project does not have access to it" while the Gemini API tests passed. Move the Vertex clients to the global location, which the pass-through router maps to aiplatform.googleapis.com, where the 3.1 family is served
* feat - allow tagging vertex JS SDK request
* add unit testing for passing headers for pass through endpoints
* fix allow using vertex_ai as the primary way for pass through vertex endpoints
* docs on vertex js pass tags
* add e2e test for vertex pass through with spend tags
* add e2e tests for streaming vertex JS with tags
* fix vertex ai testing
* stash gemini JS test
* add vertex js sdj example
* handle vertex pass through separately
* tes vertex JS sdk
* fix vertex_proxy_route
* use PassThroughStreamingHandler
* fix PassThroughStreamingHandler
* use common _create_vertex_response_logging_payload_for_generate_content
* test vertex js
* add working vertex jest tests
* move basic bass through test
* use good name for test
* test vertex
* test_chunk_processor_yields_raw_bytes
* unit tests for streaming
* test_convert_raw_bytes_to_str_lines
* run unit tests 1st
* simplify local
* docs add usage example for js
* use get_litellm_virtual_key
* add unit tests for vertex pass through
