test: harden fetch fixture synchronization

[BartWaardenburg]

Summary

• replace the scheduler-sensitive negative timeout with a staged reader and explicit channel synchronization
• keep the real TCP fixture timeout and complete-header behavior unchanged
• correct the previous implementation plan's expected file scope

Verification

• deterministic regression test failed against request-line-only behavior, then passed with complete-header reading restored
• focused CLI fetch integration tests
• pnpm check
• pnpm test

[codspeed-hq]

Merging this PR will not alter performance

✅ 188 untouched benchmarks
⏩ 76 skipped benchmarks¹

---

_{Comparing codex/harden-fetch-fixture-test (eed42f8) with main (9967c5c)}

Footnotes

1. 76 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩