indexheader: replace fixed sleep with require.Eventually in reader pool idle test

[nikolauspschuetz]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

TestReaderPool_ShouldCloseIdleLazyReaders waited for the pool's background
reaper to unload an idle reader with a fixed time.Sleep(idleTimeout * 2) (2s)
and then asserted unloadCount == 1.

The reaper goroutine ticks every idleTimeout/10 and unloads a reader once it
has been idle for idleTimeout, so the unload is an asynchronous event. This
replaces the blind sleep with require.Eventually (testify is already imported
in this file), polling unloadCount until it reaches 1 with a generous 5×
idleTimeout cap and a short tick. The surrounding loadCount/isTracking
assertions are unchanged — they are stable at the moment the unload is observed.

Net effect: both a speedup (resolves in ~1s instead of a fixed 2s of dead
time) and a de-flake (a loaded CI runner where the reaper tick slips past
the fixed 2s window no longer fails a hard timing assertion). Test-only, no
production code touched.

Verification

Ran the target test with the race detector, repeated to shake out flakiness:

go test -race -run TestReaderPool_ShouldCloseIdleLazyReaders -count=50 ./pkg/block/indexheader/
ok  github.com/thanos-io/thanos/pkg/block/indexheader  113.217s

50/50 passes, no data races. Timing comparison (-count=10, no race, same box):
old fixed-sleep 58.4s vs. poll 54.9s — the per-iteration sleep saving shows
through even though block setup dominates each iteration. gofmt clean.

---

for the record: authored with AI assistance, reviewed and verified by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.54%. Comparing base (b453e37) to head (a327d92).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8978   +/-   ##
=======================================
  Coverage   64.54%   64.54%           
=======================================
  Files         289      289           
  Lines       37359    37359           
=======================================
  Hits        24115    24115           
  Misses      11151    11151           
  Partials     2093     2093           

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