[jamesbuddrige]

Description

When Gemini on GCP Vertex AI blocks a prompt before generation, it returns promptFeedback.blockReason (for example BLOCKLIST or PROHIBITED_CONTENT) and no candidates. The OpenAI to GCP Vertex AI translator only converts candidates, and ChatCompletionResponse.Choices is omitempty, so the client receives a 200 with no choices field at all:

{"id":"...","created":1791992911,"model":"gemini-2.5-flash","object":"chat.completion","usage":{"prompt_tokens":7771,"total_tokens":7771,...}}

Nothing in that response says the prompt was rejected. OpenAI SDK clients then fail on response.choices[0], and because the block is deterministic for a given prompt, generic retry logic loops on it. We hit this in production: one blocked prompt was retried thousands of times until the caller's activity timed out.

This change returns a single choice with finish_reason: content_filter (assistant role, no content) when the prompt is blocked, in both the non-streaming and streaming paths. That matches how the translator already maps candidate-level SAFETY / BLOCKLIST / PROHIBITED_CONTENT finish reasons, and it is what OpenAI-compatible clients already check for.

Tests cover the non-streaming and streaming translation using the response shape Vertex returns for a blocked prompt, plus the helper's edge cases (no feedback, empty or unspecified reason, block reason alongside candidates). The new test cases fail without the change. make precommit and the translator package tests pass.

Special notes for reviewers (if applicable)

I used Claude Code (generative AI) to help investigate the issue and write this change. I have reviewed and understand the code and will handle review feedback.

An alternative would be to turn the block into a 4xx error. I did not do that here because translators can only rewrite the response body, so changing the status would mean extproc changes to status handling, metrics and spans. A content_filter choice keeps the fix inside the translator and consistent with the existing candidate-level handling.

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[jamesbuddrige]

/retest