feat: improve bucket assignment for MOR with bucket index

[HuangZhenQiu]

Describe the issue this Pull Request addresses

Improve Bucket assignment for MOR with bucket index

Closes #18166

Summary and Changelog

1. Improve HoodieSplitBucketAssigner with the feature parity of fix(flink): Improve splits distribution strategy for mor table w/ bucket index #18103
2. Change test cases in HoodieSplitBucketAssigner.

Impact

none

Risk Level

none

Documentation Update

none

Contributor's checklist

• Read through contributor's guide
• Enough context is provided in the sections above
• Adequate tests were added if applicable

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.51%. Comparing base (447af5a) to head (8c27e99).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18444      +/-   ##
============================================
- Coverage     68.55%   68.51%   -0.04%     
+ Complexity    27988    27984       -4     
============================================
  Files          2442     2443       +1     
  Lines        134534   134634     +100     
  Branches      16249    16278      +29     
============================================
+ Hits          92225    92248      +23     
- Misses        35038    35110      +72     
- Partials       7271     7276       +5     

Flag	Coverage Δ
common-and-other-modules	44.33% <100.00%> (-0.03%)	⬇️
hadoop-mr-java-client	44.94% <ø> (-0.05%)	⬇️
spark-client-hadoop-common	48.39% <ø> (+<0.01%)	⬆️
spark-java-tests	48.77% <ø> (-0.03%)	⬇️
spark-scala-tests	45.63% <ø> (-0.02%)	⬇️
utilities	38.32% <ø> (-0.05%)	⬇️

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

Files with missing lines	Coverage Δ
...hudi/source/split/assign/HoodieSplitAssigners.java	83.33% <100.00%> (ø)
...source/split/assign/HoodieSplitBucketAssigner.java	100.00% <100.00%> (ø)

... and 10 files with indirect coverage changes

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[hudi-bot]

CI report:

• a512db2 UNKNOWN
• 8c27e99 Azure: SUCCESS

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