[Fix] Trigger 'unlabeled' event when label is Deleted from PR (#34316)

This pull request updates the handling of issue label events in
workflows to distinguish between label additions and deletions,
introduces corresponding test cases, and extends the `IssuePayload`
structure to support this functionality.

### Enhancements to issue label event handling:
* Updated `matchIssuesEvent` in `modules/actions/workflows.go` to
differentiate between "labeled" and "unlabeled" events based on whether
labels were added or removed.
* Added a new field, `RemovedLabels`, to the `IssuePayload` struct in
`modules/structs/hook.go` to track labels that were removed during an
issue event.

### Testing improvements:
* Added `TestMatchIssuesEvent` in `modules/actions/workflows_test.go` to
cover scenarios such as label addition, label deletion, and label
clearing, ensuring the correct event type is triggered.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/actions/workflows.go

@@ -377,20 +377,28 @@ func matchIssuesEvent(issuePayload *api.IssuePayload, evt *jobparser.Event) bool
 			// Actions with the same name:
 			// opened, edited, closed, reopened, assigned, unassigned, milestoned, demilestoned
 			// Actions need to be converted:
-			// label_updated -> labeled
+			// label_updated -> labeled (when adding) or unlabeled (when removing)
 			// label_cleared -> unlabeled
 			// Unsupported activity types:
 			// deleted, transferred, pinned, unpinned, locked, unlocked
 
-			action := issuePayload.Action
-			switch action {
+			actions := []string{}
+			switch issuePayload.Action {
 			case api.HookIssueLabelUpdated:
-				action = "labeled"
+				if len(issuePayload.Changes.AddedLabels) > 0 {
+					actions = append(actions, "labeled")
+				}
+				if len(issuePayload.Changes.RemovedLabels) > 0 {
+					actions = append(actions, "unlabeled")
+				}
 			case api.HookIssueLabelCleared:
-				action = "unlabeled"
+				actions = append(actions, "unlabeled")
+			default:
+				actions = append(actions, string(issuePayload.Action))
 			}
+
 			for _, val := range vals {
-				if glob.MustCompile(val, '/').Match(string(action)) {
+				if slices.ContainsFunc(actions, glob.MustCompile(val, '/').Match) {
 					matchTimes++
 					break
 				}