chore: Fix flaky tests with deterministic runs

[gmlewis]

This PR addresses a recent race failure on Windows demonstrated here:
https://github.com/gmlewis/go-github/actions/runs/29060026853/job/86259654619

The crash was fatal error: bad g->status in ready — a Go runtime bug on Windows + -race, triggered when parallel tests with goroutines/timers complete under heavy load.

Root cause of flakiness: Two rate-limit tests relied on wall-clock timing:

• abortSleepContextCancelled: spawned a goroutine running client.Do + used time.After(10s) timeouts (2 real timers active)
• abortSleepContextCancelledClientLimit: used context.WithTimeout(10ms) — a tiny window racing with goroutine scheduling

Fix — made both fully deterministic with zero real-time dependencies:

1. abortSleepContextCancelled: Removed the errCh goroutine + time.After timeouts. Now calls client.Do synchronously; a tiny goroutine just waits for the handler's requestReceived signal then calls cancel(). The HTTP transport is synchronous, so the handler always completes before the rate-limit sleep begins — cancel fires either just before or during the sleep, both handled identically by sleepUntilResetWithBuffer's select.
2. abortSleepContextCancelledClientLimit: Replaced WithTimeout(10ms) with a pre-cancelled context (WithCancel + immediate cancel()). checkRateLimitBeforeDo → sleepUntilResetWithBuffer sees ctx.Done() already closed and returns instantly — no timing race at all.

Also fixed a copy-paste bug: "Expected 1 requests" → "Expected 0 requests".

[gmlewis]

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.51%. Comparing base (9b2c665) to head (9d464e1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #4377 +/- ##
=======================================
Coverage 97.51% 97.51% =======================================
Files 193 193 Lines 19526 19526 =======================================
Hits 19040 19040 Misses 268 268 Partials 218 218 

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[stevehipwell]

LGTM

@gmlewis as an aside did you consider using synctest to keep the timeout pattern?

[gmlewis]

@gmlewis as an aside did you consider using synctest to keep the timeout pattern?

Yes, in fact the agent started out fixing this flaky test by using synctest but then discovered it could completely remove all asynchrony and make the tests fully deterministic without changing their primary objectives without it.