Add validation on context template removal

[mgodwan]

Description

Prevent removal of system templates used via context in indices.

Related Issues

Prevent removal of system templates used via context in indices.

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 6a90a58: null

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 4a8cf94: null

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 57522c6: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.91%. Comparing base (a5590d1) to head (57522c6).
Report is 354 commits behind head on main.

Files with missing lines	Patch %	Lines
...cluster/metadata/MetadataIndexTemplateService.java	83.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #15608      +/-   ##
============================================
+ Coverage     71.85%   71.91%   +0.06%     
- Complexity    63847    63904      +57     
============================================
  Files          5257     5257              
  Lines        298543   298555      +12     
  Branches      43164    43165       +1     
============================================
+ Hits         214507   214720     +213     
+ Misses        66421    66222     -199     
+ Partials      17615    17613       -2     

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[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.

[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.

[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.