test: wait for rules-proxy watchers before changing _redirects

[paulo]

Summary

This fixes a flaky unit test from #8553. tests/unit/utils/rules-proxy.test.ts → "reloads rules when the _redirects file changes" fails intermittently on macOS:

AssertionError: expected { … } to match object { to: '/newer' }
-   "to": "/newer",
+   "to": "/new",

It failed in 2 of the last 10 Unit (macOS-latest, 24) runs (on #8558's branch at 81a5cc3b3 and 57eb7cd32). Neither failure has anything to do with that branch's changes.

Cause: createRewriter starts its chokidar watchers without waiting for them to be ready. The test makes one request and then rewrites _redirects. When the write lands before the watcher has armed its file watch, the change is never reported:

• the failing runs wait the full 5s waitFor timeout instead of the usual ~200ms for the whole file;
• the "Reloading redirect rules" log line never appears.

On fast machines the first request (which builds the matcher) outlasts watcher setup, so the test usually passes.

Fix: the rewriterFor test helper waits for every watcher's ready event before returning. createRewriter returns synchronously right after starting the watchers, and ready only fires after filesystem I/O, so the listener is always attached in time and can't hang. Production code is unchanged. A person editing _redirects during netlify dev does so long after the watchers are ready.

How this was verified

• The file passes 9/9 tests, 15 runs in a row, and still finishes in about 0.4s.
• The full unit suite with coverage passes. The generate-autocompletion failure also happens on main.
• Not reproduced locally: I couldn't make the flake happen here, even under CPU load or with chokidar on its own. So there's no local before-and-after; repeated green macOS runs are the real confirmation.

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• tests/unit/utils/rules-proxy.test.ts

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📊 Benchmark results

Comparing with 086fbfc

• Dependency count: 1,018 (no change)
• Package size: 381 MB (no change)
• Number of ts-expect-error directives: 331 (no change)

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commit: b9ae9fd