[fix][test] Fix flaky PersistentTopicTest.testBacklogCursor

[merlimat]

Flaky test failure

java.lang.AssertionError: expected [false] but found [true]
	at org.testng.Assert.fail(Assert.java:110)
	at org.testng.Assert.failNotEquals(Assert.java:1577)
	at org.testng.Assert.assertFalse(Assert.java:78)
	at org.testng.Assert.assertFalse(Assert.java:88)
	at org.apache.pulsar.broker.service.PersistentTopicTest.testBacklogCursor(PersistentTopicTest.java:2019)

Summary

• Fix flaky PersistentTopicTest.testBacklogCursor which expects checkBackloggedCursors() to deactivate cursors with backlog exceeding the threshold.
• The ManagedLedgerConfig default has cacheEvictionByExpectedReadCount=true, which causes checkBackloggedCursor() to return early without processing any cursors. The test was passing only when a previous test happened to change this flag.
• Set cacheEvictionByExpectedReadCount=false on the ledger config so that backlog-based cursor deactivation is actually exercised.

Documentation

• doc-not-needed
  (Your PR doesn't need any doc update)

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.73%. Comparing base (7f6bc23) to head (7912556).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #25348       +/-   ##
=============================================
+ Coverage     37.45%   72.73%   +35.27%     
- Complexity    13161    33869    +20708     
=============================================
  Files          1897     1954       +57     
  Lines        150557   154763     +4206     
  Branches      17215    17718      +503     
=============================================
+ Hits          56398   112564    +56166     
+ Misses        86428    33173    -53255     
- Partials       7731     9026     +1295     

Flag	Coverage Δ
inttests	25.88% <ø> (-0.17%)	⬇️
systests	22.56% <ø> (+0.06%)	⬆️
unittests	73.70% <ø> (+39.53%)	⬆️

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see 1417 files with indirect coverage changes

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