[Backport 2.x] Correct RHLC searchTemplate & mtermVectors endpoints to have leading slash (#14465)

[Xtansia]

Description

Backport 9675c4f from #14465

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

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

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[dblock]

Let’s try to get this into 2.15 as well. I labeled this with a backport.

[github-actions]

❌ Gradle check result for 6d28617: 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 6d28617: 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 b1529d7: 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?

[Xtansia]

Failing test is org.opensearch.action.admin.indices.create.RemoteCloneIndexIT.testCreateCloneIndexLowPriorityRateLimit which has reports of other failures in #14292

[github-actions]

✅ Gradle check result for b1529d7: SUCCESS

[opensearch-trigger-bot]

The backport to 2.15 failed:

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

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.15 2.15
# Navigate to the new working tree
pushd ../.worktrees/backport-2.15
# Create a new branch
git switch --create backport/backport-14480-to-2.15
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 a9d806a6c4fa9b10d1a3344f446134d82dece351
# Push it to GitHub
git push --set-upstream origin backport/backport-14480-to-2.15
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.15

Then, create a pull request where the base branch is 2.15 and the compare/head branch is backport/backport-14480-to-2.15.