fix(ai-gateway): standardize stream error payloads

[chrarnoldus]

Summary

• remove the non-standard vercel_request_id property from JSON response-read errors
• remove the same property from Chat Completions, Messages, and Responses SSE error objects
• retain the request ID suffix in each human-readable error message and retain internal request-ID logging
• update focused assertions to require that streamed error objects omit the custom field

Compatibility

Vercel AI SDK v6 parses protocol-native error shapes:

• OpenAI-compatible errors use error.message with optional type, param, and code
• Anthropic Messages error events use error.type and error.message
• OpenAI Responses error events use their standard type, code, and message fields

The custom vercel_request_id member is unnecessary for correlation because the same value remains embedded in message, and omitting it keeps the emitted objects within those protocol shapes.

Reference: https://github.com/vercel/ai/tree/release-v6.0

Verification

• pnpm --filter web exec jest --runInBand --forceExit src/lib/rewriteModelResponse.test.ts (72 tests passed)
• pnpm exec oxfmt --list-different apps/web/src/lib/rewriteModelResponse.ts apps/web/src/lib/rewriteModelResponse.test.ts
• git diff --check

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

High confidence: the diff is a pure removal of the non-standard vercel_request_id member from the JSON read-error body and from the Chat Completions, Messages, and Responses SSE error objects, with request-ID correlation preserved in every message string and in internal errorExceptInTest/logExceptInTest logging, and no remaining references to the dropped ResponseReadError.vercelRequestId field.

Files Reviewed (2 files)

• apps/web/src/lib/rewriteModelResponse.ts
• apps/web/src/lib/rewriteModelResponse.test.ts

Notes (out of diff scope)

• apps/web/src/lib/ai-gateway/providers/upstream-request.ts:161 and :185 still emit vercel_request_id for pre-response failures (client disconnect / upstream fetch failure), so the field remains part of the gateway's error surface for those paths. Not changed by this PR; flagged only as a consistency follow-up if full standardization is the goal.

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_{Reviewed by claude-opus-5 · Input: 24 · Output: 6K · Cached: 530.2K}

_{Review guidance: REVIEW.md from base branch main}