[fix][client] Skip processing messages in the listener when the consumer has been closed

[lhotari]

Fixes #23193

Motivation

There's currently a NPE that happens when a multi-topic consumer has been closed and there are pending messages buffered in the message listener executor's queue.

Modifications

• skip processing messages in the listener when the consumer has already been closed
• make unAckedMessageTracker final and don't set it to null while closing a multi-topic consumer
• add tests that validate that processing of messages are skipped when the consumer has been closed
• add TestLogAppender that can collect Log4J events during the test run. It's useful for asserting that the logs don't contain any errors.

Documentation

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.29%. Comparing base (807dcaf) to head (eeef9dc).

Files with missing lines	Patch %	Lines
...va/org/apache/pulsar/client/impl/ConsumerBase.java	33.33%	7 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #25006      +/-   ##
============================================
+ Coverage     74.27%   74.29%   +0.01%     
- Complexity    33563    33680     +117     
============================================
  Files          1920     1920              
  Lines        150279   150293      +14     
  Branches      17448    17454       +6     
============================================
+ Hits         111618   111655      +37     
+ Misses        29768    29719      -49     
- Partials       8893     8919      +26     

Flag	Coverage Δ
inttests	26.16% <0.00%> (?)
systests	22.85% <6.66%> (+<0.01%)	⬆️
unittests	73.83% <33.33%> (-0.05%)	⬇️

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

Files with missing lines	Coverage Δ
...he/pulsar/client/impl/MultiTopicsConsumerImpl.java	76.42% <ø> (-0.59%)	⬇️
...va/org/apache/pulsar/client/impl/ConsumerBase.java	74.17% <33.33%> (-1.04%)	⬇️

... and 86 files with indirect coverage changes

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[dao-jun]

LGTM