fix MLTaskState enum serialization errors

[Zhangxunmt]

Description

Fix the search query enum serialization errors. The enum cannot be serialized and needs to be converted to String.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[dhrubo-os]

Can we add a unit test for this?

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.82%. Comparing base (45002a7) to head (ed7264d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ain/java/org/opensearch/ml/task/MLTaskManager.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #4158   +/-   ##
=========================================
  Coverage     81.81%   81.82%           
- Complexity     8866     8867    +1     
=========================================
  Files           762      762           
  Lines         38167    38167           
  Branches       4264     4264           
=========================================
+ Hits          31228    31231    +3     
+ Misses         5111     5109    -2     
+ Partials       1828     1827    -1     

Flag	Coverage Δ
ml-commons	81.82% <0.00%> (+<0.01%)	⬆️

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