ci: leave comment and abort early if commit already exists on target branch in backport workflow (#6326)

Follow up on #6317:
Fixes confusing "merge conflicts detected" message when commit already
exists on target branch (e.g., PRs #6294 and #6307).

## Changes

- Added check to detect if merge commit already exists on target branch
before attempting cherry-pick
- New failure reason `already-exists` for this case
- Clear comment message: "Commit already exists on branch, no backport
needed" instead of confusing "Merge conflicts detected" with empty file
list

## Before

When a commit already existed on the target branch, users would see:
> @user Backport to `core/1.30` failed: Merge conflicts detected.
> Please manually cherry-pick commit `abc123` to the `core/1.30` branch.
> <details><summary>Conflicting files</summary>
> 
> </details>

This was confusing because there were no actual conflicts - the commit
was already present.

## After

Users now see:
> @user Commit `abc123` already exists on branch `core/1.30`. No
backport needed.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6326-ci-leave-comment-and-abort-early-if-commit-already-exists-on-target-branch-in-backport-w-2996d73d36508167aff3e6783e832c74)
by [Unito](https://www.unito.io)

Author: christian-byrne

.github/workflows/pr-backport.yaml

@@ -234,6 +234,14 @@ jobs:
               continue
             fi
 
+            # Check if commit already exists on target branch
+            if git branch -r --contains "${MERGE_COMMIT}" | grep -q "origin/${TARGET_BRANCH}"; then
+              echo "::notice::Commit ${MERGE_COMMIT} already exists on ${TARGET_BRANCH}, skipping backport"
+              FAILED="${FAILED}${TARGET_BRANCH}:already-exists "
+              echo "::endgroup::"
+              continue
+            fi
+
             # Create backport branch
             git checkout -b "${BACKPORT_BRANCH}" "origin/${TARGET_BRANCH}"
 
@@ -328,6 +336,9 @@ jobs:
             if [ "${reason}" = "branch-missing" ]; then
               gh pr comment "${PR_NUMBER}" --body "@${PR_AUTHOR} Backport failed: Branch \`${target}\` does not exist"
 
+            elif [ "${reason}" = "already-exists" ]; then
+              gh pr comment "${PR_NUMBER}" --body "@${PR_AUTHOR} Commit \`${MERGE_COMMIT}\` already exists on branch \`${target}\`. No backport needed."
+
             elif [ "${reason}" = "conflicts" ]; then
               # Convert comma-separated conflicts back to newlines for display
               CONFLICTS_LIST=$(echo "${conflicts}" | tr ',' '\n' | sed 's/^/- /')