Fix race in keepalive drain tests

[Copilot]

CI detected a race in TestDrainingReadCloser: tests reassigned process-global io.Discard while net/http background drain goroutines could read it.

• Test isolation

  • Add an optional drain writer to drainingReadCloser.
  • Preserve io.Discard as the production default.

• Race-free assertions

  • Inject per-test buffers to verify drained output without mutating global state.

discard := d.discard
if discard == nil {
    discard = io.Discard
}
_, _ = io.Copy(discard, d.rdr)

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.16%. Comparing base (4fb7297) to head (d9bc418).
⚠️ Report is 8 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
client/keepalive.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #529      +/-   ##
==========================================
- Coverage   87.17%   87.16%   -0.02%     
==========================================
  Files          65       65              
  Lines        4749     4752       +3     
==========================================
+ Hits         4140     4142       +2     
- Misses        599      600       +1     
  Partials       10       10              

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