Correct RHLC searchTemplate & mtermVectors endpoints to have leading slash

[Xtansia]

Description

These non-absolute paths (ie. no leading /) are being passed verbatim to the HTTP request which results in a technically invalid request start-line of GET _render/template HTTP/1.1 which should be GET /_render/template HTTP/1.1. OpenSearch itself is lenient and happily serves this request, however in cases of stricter intermediaries such as proxies or load-balances as in Amazon OpenSearch Service, this request ends up denied with a 400 Bad Request.

Ideally this would be solved globally in RestClient (#14423) however this in the minimal easily backportable fix for now.

Related Issues

Relates to #14423

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 a85a318: 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]

❌ Gradle check result for 1b710de: 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]

✅ Gradle check result for 319b5ed: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.75%. Comparing base (b15cb0c) to head (319b5ed).
Report is 463 commits behind head on main.

❗ Current head 319b5ed differs from pull request most recent head a9ae678

Please upload reports for the commit a9ae678 to get more accurate results.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14465      +/-   ##
============================================
+ Coverage     71.42%   71.75%   +0.33%     
- Complexity    59978    62060    +2082     
============================================
  Files          4985     5118     +133     
  Lines        282275   291976    +9701     
  Branches      40946    42211    +1265     
============================================
+ Hits         201603   209498    +7895     
- Misses        63999    65164    +1165     
- Partials      16673    17314     +641     

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[github-actions]

❕ Gradle check result for a9ae678: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-14465-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9675c4f6ec0d412993ef361bce44a8b789bff27b
# Push it to GitHub
git push --set-upstream origin backport/backport-14465-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-14465-to-2.x.