Fix NPE when creating a match-all composable index template without an inline template

[llilyy]

Description

Fixes a NullPointerException when creating a composable index template with a match-all pattern (*) in index_patterns and without an inline template block:

PUT /_index_template/my-catch-all
{
  "index_patterns": ["*"],
  "priority": 0,
  "composed_of": ["my-defaults"]
}

{
  "error": {
    "type": "null_pointer_exception",
    "reason": "Cannot invoke \"org.opensearch.cluster.metadata.Template.settings()\" because the return value of \"org.opensearch.cluster.metadata.ComposableIndexTemplate.template()\" is null"
  },
  "status": 500
}

Request#validate() checked the index.hidden restriction for global templates by calling indexTemplate.template().settings() without null checks. Since the template block is optional, both template() and its settings() can be null, which caused the NPE. With this change, the index.hidden validation only runs when an inline template with settings is actually present, so a match-all, composed_of-only template can be created.

The same bug existed in Elasticsearch and was fixed in 7.10.3 (elastic/elasticsearch#67310).

Added unit tests covering a global template without an inline template and one with an inline template but null settings.

Related Issues

Resolves #22420

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]

PR Reviewer Guide 🔍

(Review updated until commit c579dbd)

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🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

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⚡ No major issues detected

[github-actions]

Persistent review updated to latest commit c9b04de

[github-actions]

✅ Gradle check result for c9b04de: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.42%. Comparing base (0a75b13) to head (c9b04de).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...template/put/PutComposableIndexTemplateAction.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #22421      +/-   ##
============================================
- Coverage     73.48%   73.42%   -0.07%     
+ Complexity    76464    76391      -73     
============================================
  Files          6101     6102       +1     
  Lines        346488   346525      +37     
  Branches      49871    49877       +6     
============================================
- Hits         254620   254432     -188     
- Misses        71599    71805     +206     
- Partials      20269    20288      +19     

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[github-actions]

Persistent review updated to latest commit 94c1dc6

[github-actions]

❌ Gradle check result for 94c1dc6: FAILURE

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]

Persistent review updated to latest commit e273298

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❌ Gradle check result for e273298: 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]

Persistent review updated to latest commit c579dbd

[github-actions]

❌ Gradle check result for c579dbd: FAILURE

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?