fix ppl allow list bug

[xinyual]

Description

[Describe what this change achieves]

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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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.37%. Comparing base (2b76e3c) to head (4b5e852).
Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
.../main/java/org/opensearch/agent/tools/PPLTool.java	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #512      +/-   ##
============================================
+ Coverage     81.78%   82.37%   +0.58%     
- Complexity      193      355     +162     
============================================
  Files            11       17       +6     
  Lines           961     1685     +724     
  Branches        137      240     +103     
============================================
+ Hits            786     1388     +602     
- Misses          121      203      +82     
- Partials         54       94      +40     

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