Fixing bug in transform API validation to support index patterns

[tandonks]

Description

Fixing #1224

Issues Resolved

Resolves #1224

Check List

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[Hailong-am]

can we have a test to cover this change?

[shiv0408]

Thanks for fixing and adding an integration test 🙌

[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/index-management-dashboards-plugin/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/index-management-dashboards-plugin/backport-2.x
# Create a new branch
git switch --create backport/backport-1237-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bed34ddf5e709ba75c77412bb36231c0c953b1ca
# Push it to GitHub
git push --set-upstream origin backport/backport-1237-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/index-management-dashboards-plugin/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-1237-to-2.x.