Vertex AI Gemma's chatCompletions wrapper does not understand the
context_management parameter (an Anthropic / OpenAI Responses API
concept). When callers route this field to a Gemma deployment (e.g.
through allowed_openai_params or proxy passthrough), the upstream
endpoint would reject the request with an unknown-field error.
Drop context_management in VertexGemmaConfig.transform_request,
matching the existing pattern used for stream and stream_options.
Adds a direct transform_request unit test plus an acompletion-level
test that exercises the realistic allowed_openai_params path.
Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Mateo Wang <mateo-berri@users.noreply.github.com>
