fix(ai-gateway): retain streamed upstream responses

[marius-kilocode]

The proxy returns upstream responses by constructing a new NextResponse around response.body. For fetched responses, runtime cleanup remains associated with the original Response object. Passing only the body to a new response does not transfer that ownership. If the original response becomes unreachable before the downstream reader is acquired, garbage collection can cancel the returned branch even while sibling clones continue consuming the upstream stream.

This is especially hazardous in the shared proxy path, where metrics, usage, and optional request logging create independent tee branches before the original body is handed to Next.js. A sibling branch completing therefore does not protect the client-facing leaf from cleanup tied to the original fetched response.

The response wrapper now uses a lazy identity ReadableStream that keeps the original response strongly reachable for the full lifetime of the returned body:

• The source reader is acquired only on the first downstream pull, preserving lazy streaming and backpressure.
• Chunks pass through unchanged without buffering, parsing, or cloning.
• The original response remains reachable until source EOF, source error, or downstream cancellation.
• EOF closes the returned stream and releases both the reader lock and retained owner.
• Source errors propagate after deterministic cleanup.
• Downstream cancellation propagates before or after the first pull.
• A cancellation flag prevents a pending pull from closing or erroring a stream that the consumer has already cancelled.
• Bodyless responses retain a null body, and status, status text, and safe-header behavior remain unchanged.

The fix applies at the common wrapInSafeNextResponse boundary used by streaming model responses and other direct proxy routes. It does not change provider selection, response rewriting, retry behavior, protocol interpretation, or completion semantics.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

All 3 previously flagged warnings are resolved: the cancelled flag guards prevent controller.close()/controller.error() from being called on an already-cancelled stream, and both Node.js and undici version pins have been removed from the GC fixture.

Previously Flagged Issues — All Resolved

File	Line	Issue	Status
apps/web/src/lib/ai-gateway/llm-proxy-helpers.ts	317–325	controller.close() / controller.error() could throw on already-cancelled stream	✅ Fixed — if (cancelled) return guards added before both calls
apps/web/src/lib/ai-gateway/wrap-safe-next-response.gc.ts	—	Hard-pinned Node.js version caused GC test to silently skip on version upgrades	✅ Fixed — version gate removed; test now runs unconditionally
apps/web/src/lib/ai-gateway/wrap-safe-next-response.gc.ts	—	Hard-pinned undici version would throw on upgrades causing confusing CI failures	✅ Fixed — undici assertion removed

Files Reviewed (3 files)

• apps/web/src/lib/ai-gateway/llm-proxy-helpers.ts — 0 issues
• apps/web/src/lib/ai-gateway/wrap-safe-next-response.gc.ts — 0 issues
• apps/web/src/lib/ai-gateway/llm-proxy-helpers.test.ts — 0 issues (new cancellation-during-pull test case correctly validates the fix)

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_{Reviewed by claude-4.6-sonnet-20260217 · 280,524 tokens}

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