Bug fixes and unit tests

[kaituo]

Description

First, this PR adds an empty string check in PriorityCache.
Second, in EntityColdStarter, this PR clears samples after using them for training.
Third, this PR removes unused fields and local variables in ModelManager and BulkUtil.
Fourth, this PR decreases the concurrent preview from 5 to 2, with a conservative purpose to reduce potential GC.
Fifth, this PR finishes unit tests owed due to my previous PRs. 99% code of the PR is about tests.

Testing done:

1. Added unit tests.
2. made sure the basic e2e workflow still works.

Check List

• Commits are signed per the DCO using --signoff

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[codecov-commenter]

Codecov Report

Merging #177 (1ddce7d) into main (878918e) will increase coverage by 2.46%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##               main     #177      +/-   ##
============================================
+ Coverage     67.66%   70.12%   +2.46%     
- Complexity     2885     2995     +110     
============================================
  Files           270      270              
  Lines         14104    14088      -16     
  Branches       1418     1414       -4     
============================================
+ Hits           9543     9879     +336     
+ Misses         3973     3590     -383     
- Partials        588      619      +31     

Flag	Coverage Δ
plugin	70.12% <75.00%> (+2.46%)	⬆️

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

Impacted Files	Coverage Δ
.../java/org/opensearch/ad/caching/PriorityCache.java	68.05% <0.00%> (ø)
...opensearch/ad/indices/AnomalyDetectionIndices.java	70.00% <ø> (+19.68%)	⬆️
...c/main/java/org/opensearch/ad/ml/ModelManager.java	85.47% <ø> (+3.85%)	⬆️
...ad/transport/handler/MultiEntityResultHandler.java	91.89% <ø> (+78.37%)	⬆️
src/main/java/org/opensearch/ad/util/BulkUtil.java	71.42% <ø> (+54.28%)	⬆️
src/main/java/org/opensearch/ad/util/MathUtil.java	100.00% <ø> (ø)
...rg/opensearch/ad/util/ThrowingSupplierWrapper.java	100.00% <ø> (+60.00%)	⬆️
...n/java/org/opensearch/ad/ml/EntityColdStarter.java	81.51% <100.00%> (+2.60%)	⬆️
...pensearch/ad/settings/AnomalyDetectorSettings.java	100.00% <100.00%> (ø)
...arch/ad/transport/EntityResultTransportAction.java	86.44% <100.00%> (-0.12%)	⬇️
... and 23 more