Add unit test for CompletedConsumingOfflineSegmentAssignment

[J-HowHuang]

Description

This PR adds an unit test covering #16348
cc @Jackie-Jiang

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.40%. Comparing base (1a476de) to head (1b93609).
Report is 487 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (1a476de) and HEAD (1b93609). Click for more details.

HEAD has 68 uploads less than BASE

Flag	BASE (1a476de)	HEAD (1b93609)
java-21	10	1
unittests	6	1
unittests1	3	1
skip-bytebuffers-true	4	0
temurin	15	1
unittests2	3	0
skip-bytebuffers-false	8	0
java-11	5	0
integration	9	0
integration2	3	0
integration1	3	0
custom-integration1	3	0

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #16400      +/-   ##
============================================
- Coverage     62.90%   56.40%   -6.51%     
+ Complexity     1386      699     -687     
============================================
  Files          2867     2321     -546     
  Lines        163354   128441   -34913     
  Branches      24952    20468    -4484     
============================================
- Hits         102755    72442   -30313     
+ Misses        52847    50001    -2846     
+ Partials       7752     5998    -1754     

Flag	Coverage Δ
custom-integration1	?
integration	?
integration1	?
integration2	?
java-11	?
java-21	56.40% <ø> (-6.43%)	⬇️
skip-bytebuffers-false	?
skip-bytebuffers-true	?
temurin	56.40% <ø> (-6.51%)	⬇️
unittests	56.40% <ø> (-6.50%)	⬇️
unittests1	56.40% <ø> (+0.58%)	⬆️
unittests2	?

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