Fix/streamed response termination 2#3533
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A streamed SSR response commits its headers (typically a 200, with no Content-Length) as soon as Next.js flushes them, well before the body finishes. If the render then fails *after* that point, response finalization had no error path: - a render that aborts the response mid-stream leaves the body hung open forever (the @fastly/http-compute-js source stream never closes), and - a render that throws hits the catch which sets statusCode=500 and writes "Internal Server Error" - but the status change is a no-op once headers are sent, so the client receives a 200 with the error string concatenated onto partial HTML (a garbled but "complete" response). Both reach the edge as an unparseable HTTP message, which ATS reports as a 502 (ats_status_502_invalid_http_response). Exposure scales with how long the response stream stays open, i.e. with pages that do a lot of async work during render. Fix: - When the handler fails after headers are committed, end the response cleanly (so buffered bytes flush in order) and record the error instead of appending "Internal Server Error". - Replace the body TransformStream (flush-only, no error path) with a ReadableStream that surfaces a recorded render error to the platform and, if the response was destroyed without a clean end, errors promptly instead of hanging. The happy path is unchanged (full body, then background work, then close). Adds regression tests that mirror the finalization pipeline. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Some tidying up of #3532 (opening as separate PR so it's not from fork and let tests fully run)
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