Return 409 for PUT index when index exists

[MdTanwer]

Summary

• Return HTTP 409 (Conflict) instead of 400 when PUT <index> is called for an index that already exists
• Map ResourceAlreadyExistsException to RestStatus.CONFLICT
• Update related unit tests

Fixes #20072

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ Recommended focus areas for review Behavior Change Changing the HTTP status returned by ResourceAlreadyExistsException from 400 to 409 is a breaking API change. Any client code, integration tests, or downstream tooling that currently relies on receiving 400 for duplicate index creation (or other paths that throw this exception, e.g., index template/alias creation, snapshot restore) will see a different status code. While 409 is more semantically correct per the linked issue, this should be called out in release notes/breaking-changes documentation, and other test suites that assert 400 for these flows should be audited. return RestStatus.CONFLICT;

[github-actions]

✅ Gradle check result for 06c6e7e: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.41%. Comparing base (319d78a) to head (06c6e7e).

Additional details and impacted files

@@            Coverage Diff            @@
##               main   #22354   +/-   ##
=========================================
  Coverage     73.40%   73.41%           
- Complexity    76075    76111   +36     
=========================================
  Files          6076     6076           
  Lines        345500   345500           
  Branches      49732    49732           
=========================================
+ Hits         253608   253638   +30     
+ Misses        71707    71635   -72     
- Partials      20185    20227   +42     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.