Fix cherry-pick workflow for fork PRs and comment on success

[hanwen-cluster]

Description of changes

The cherry-pick workflow failed with "fatal: bad object " when the source PR came from a fork in another account. The job checks out the base repo and only fetches the target branch, so the PR's commits were never present locally and the cherry-pick could not resolve them.

Fetch refs/pull//head before cherry-picking so the PR's original commits are available regardless of which fork they came from.

Also comment on the original PR when a cherry-pick succeeds, linking to the newly opened PR. This mirrors the existing failure comment and gives a clear per-target-branch status when a PR is backported to multiple branches.

References

• Link to impacted open issues.
• Link to related PRs in other packages (i.e. cookbook, node).
• Link to documentation useful to understand the changes.

Checklist

• Make sure you are pointing to the right branch.
• If you're creating a patch for a branch other than develop add the branch name as prefix in the PR title (e.g. [release-3.6]).
• Check all commits' messages are clear, describing what and why vs how.
• Make sure to have added unit tests or integration tests to cover the new/modified code.
• Check if documentation is impacted by this change.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.