Avoid using unclosing prefetch streams in the browser#89610
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Previously, `createPrefetchResponseStream` intentionally never called `controller.close()` on the wrapper stream, to prevent React Flight from erroring on unresolved references (PPR dynamic holes). However, Chrome and Firefox keep unclosed ReadableStreams with pending reads as native GC roots, preventing the stream — and the entire FlightResponse captured in the `reader.read().then(progress)` closure chain — from being collected. Now that React Flight supports `unstable_allowPartialStream`, we can close the stream normally. Flight will mark unresolved chunks as "halted" instead of erroring, which is the correct behavior for prefetch responses. Also removes the now-unnecessary `createUnclosingPrefetchStream` wrapper from the legacy prefetch path in `fetch-server-response.ts`.
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Follow-up to #89610. Adds an e2e test that detects linear heap growth caused by unclosed prefetch response streams. The test cycles through mounting/unmounting links to trigger repeated prefetches and LRU evictions, then uses CDP heap profiling with linear regression to verify the heap plateaus rather than growing steadily.
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…9756) Follow-up to #89610. Adds an e2e test that detects linear heap growth caused by unclosed prefetch response streams. The test cycles through mounting/unmounting links to trigger repeated prefetches and LRU evictions, then uses CDP heap profiling with linear regression to verify the heap plateaus rather than growing steadily. 
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Previously,
createPrefetchResponseStreamintentionally never calledcontroller.close()on the wrapper stream, to prevent React Flight from erroring on unresolved references (dynamic holes). However, Chrome and Firefox keep unclosed ReadableStreams with pending reads as native GC roots, preventing the stream — and the entire FlightResponse captured in thereader.read().then(progress)closure chain — from being garbage-collected.Now that React Flight supports
unstable_allowPartialStreamas an option forcreateFromReadableStream(facebook/react#35731), we can close the stream normally. Flight will mark unresolved chunks as "halted" instead of erroring, which is the correct behavior for prefetch responses.Also removes the now-unnecessary
createUnclosingPrefetchStreamwrapper from the legacy prefetch path infetch-server-response.ts.closes #89485