[fix][broker] Handle missing replicator during snapshot request processing

[Denovo1998]

Fixes #xyz

Main Issue: #xyz

PIP: #xyz

Motivation

A race condition can happen when a replicated subscription snapshot request is being processed while the corresponding replicator is removed concurrently. In this case, topic.getReplicators().get(...) may return null, which can lead to an exception path instead of graceful handling. This affects broker reliability under concurrent replication policy updates and marker handling.

Modifications

• Added a null-check in ReplicatedSubscriptionsController#receivedSnapshotRequest for the source-cluster replicator.
• When the replicator is missing, the broker now logs a warning with topic, snapshot id, and source cluster, then safely returns.
• Kept the existing reconnect path (startReplProducers) for non-null but disconnected replicators.
• Added ReplicatedSubscriptionsControllerTest#testSnapshotRequestWhenReplicatorRemovedConcurrentlyDoesNotThrow.
• The test deterministically reproduces the race by blocking replicators.get(...), removing the replicator through PersistentTopic#removeReplicator, then resuming marker handling.
• The test verifies replicator termination and cursor cleanup are invoked through the production removal path, and confirms marker handling completes without throwing.

Verifying this change

• Make sure that the change passes the CI checks.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: Denovo1998#22

[lhotari]

LGTM

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.63%. Comparing base (24eba10) to head (7ceb553).
⚠️ Report is 69 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #25266       +/-   ##
=============================================
+ Coverage     37.65%   72.63%   +34.97%     
- Complexity    13351    34417    +21066     
=============================================
  Files          1902     1959       +57     
  Lines        151237   155442     +4205     
  Branches      17238    17739      +501     
=============================================
+ Hits          56954   112900    +55946     
+ Misses        86577    33541    -53036     
- Partials       7706     9001     +1295     

Flag	Coverage Δ
inttests	25.60% <0.00%> (-0.29%)	⬇️
systests	22.43% <0.00%> (-0.02%)	⬇️
unittests	73.60% <100.00%> (+39.26%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../persistent/ReplicatedSubscriptionsController.java	75.00% <100.00%> (+75.00%)	⬆️

... and 1420 files with indirect coverage changes

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