feat(ai-gateway): log Vercel upstream provider id as upstream_id

[chrarnoldus]

Problem

The Vercel AI Gateway reports the upstream provider's own id in
provider_metadata.gateway.routing.modelAttempts[].providerAttempts[].providerRequestId
(and providerResponseId), e.g. Fireworks' chatcmpl-ccebc94c526d4f8797cfe00023478a9a.

microdollar_usage_metadata.upstream_id was only ever populated from the
OpenRouter-style GET /generation?id=… lookup, and that lookup is skipped for
healthy Vercel requests (they already report cost and finalProvider), so the
upstream provider id was never logged.

Change

• VercelProviderAttempt now models providerRequestId / providerResponseId.
• New extractVercelUpstreamId() in processUsage.shared.ts, sharing the
  serving-attempt selection with extractVercelIsByok() via
  findVercelServingProviderAttempt().
• Wired into all six Vercel parse paths that previously hardcoded
  upstream_id: null: chat_completions (stream + JSON), messages (stream +
  JSON), responses (stream + JSON).
• processTokenData no longer clobbers a response-derived upstream_id with a
  null from the /generation merge.

Verification

• jest src/lib/ai-gateway/processUsage — 74 passed, including three new
  chat_completions tests (streamed id, missing id stays null, non-streamed id).
• The vercel-messages/vercel-responses approval fixtures already contained
  providerRequestId; their approved output now shows the real upstream ids
  instead of null.
• tsgo --noEmit, pnpm lint, pnpm format.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Reviewed the Vercel AI Gateway upstream_id extraction change; the attempt-selection logic, all six wired call sites, and the processTokenData merge fix are consistent and covered by new/updated tests with no issues found in the changed code.

Files Reviewed (9 files)

• apps/web/src/lib/ai-gateway/processUsage.messages.ts
• apps/web/src/lib/ai-gateway/processUsage.responses.ts
• apps/web/src/lib/ai-gateway/processUsage.shared.ts
• apps/web/src/lib/ai-gateway/processUsage.test.ts
• apps/web/src/lib/ai-gateway/processUsage.ts
• apps/web/src/lib/ai-gateway/processUsage.types.ts
• apps/web/src/tests/sample/vercel-messages.log.resp.sse.approved.json
• apps/web/src/tests/sample/vercel-responses.log.resp.json.approved.json
• apps/web/src/tests/sample/vercel-responses.log.resp.sse.approved.json

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