Add workflow to detect backport conflicts before merging

[lbhsos]

Description

This PR introduces a new GitHub Actions workflow, Backport Conflict Check.
The workflow automatically verifies whether a pull request can be cleanly cherry-picked into a branch labeled with a backport <version> tag.

Key features:

• Detects backport <version> label on PR.
• Fetches the corresponding upstream target branch.
• Attempts git cherry-pick of the PR’s head commit.
• Reports conflicts back to the PR as a comment when detected.

Related Issues

Resolves #12097

Check List

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

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[cwperks]

@lbhsos interesting PR :). Did you test this on your fork? I'd be curious to see an example comment that it leaves on a PR.

Can you also please fix the DCO check by signing off on your commits? git commit -m "message here" --signoff

[lbhsos]

@cwperks
Yes, here's my test on my fork.
I've also fixed the DCO check. Thank you for the heads-up!

[opensearch-trigger-bot]

This PR is stalled because it has been open for 30 days with no activity.