[ISSUE #9980] Skip invalid POP records when consumer group does not exist (#9980)

[Gautam-aman]

What does this PR do?

Skip invalid POP records when the consumer group does not exist, preventing
unnecessary checkpoint and retry message generation.

Why is it needed?

Currently, POP checkpoints may still be written even if the consumer group
is absent, leading to invalid retries and wasted resources.

How was this tested?

Verified locally with broker module build and full reactor build.
All quality checks (SpotBugs, Checkstyle) pass.

Related Issue

Fixes #9980

[Gautam-aman]

Local build and full reactor completed successfully on JDK 17.
No functional changes outside POP broker logic.

[lizhimins]

must remove flattened-pom.xml from your pr

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.70%. Comparing base (7cc0774) to head (45cccfe).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...cketmq/broker/processor/PopBufferMergeService.java	0.00%	9 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #9982      +/-   ##
=============================================
- Coverage      48.84%   48.70%   -0.15%     
+ Complexity     12773    12729      -44     
=============================================
  Files           1324     1324              
  Lines          96405    96414       +9     
  Branches       12434    12436       +2     
=============================================
- Hits           47093    46962     -131     
- Misses         43571    43691     +120     
- Partials        5741     5761      +20     

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[Gautam-aman]

Thanks for pointing this out all generated .flattened-pom.xml files have been removed from the PR.