Prevent re-review and dismiss review actions on closed and merged PRs (#30065)

Resolves #29965.

---
Manually tested this by:
- Following the
[installation](https://docs.gitea.com/next/installation/install-with-docker#basics)
guide (but built a local Docker image instead)
- Creating 2 users, one who is the `Owner` of a newly-created repository
and the other a `Collaborator`
- Had the `Collaborator` create a PR that the `Owner` reviews
- `Collaborator` resolves conversation and `Owner` merges PR

And with this change we see that we can no longer see re-request review
button for the `Owner`:

<img width="1351" alt="Screenshot 2024-03-25 at 12 39 18 AM"
src="https://github.com/go-gitea/gitea/assets/60799661/bcd9c579-3cf7-474f-a51e-b436fe1a39a4">

models/issues/review.go

@@ -66,6 +66,23 @@ func (err ErrNotValidReviewRequest) Unwrap() error {
 	return util.ErrInvalidArgument
 }
 
+// ErrReviewRequestOnClosedPR represents an error when an user tries to request a re-review on a closed or merged PR.
+type ErrReviewRequestOnClosedPR struct{}
+
+// IsErrReviewRequestOnClosedPR checks if an error is an ErrReviewRequestOnClosedPR.
+func IsErrReviewRequestOnClosedPR(err error) bool {
+	_, ok := err.(ErrReviewRequestOnClosedPR)
+	return ok
+}
+
+func (err ErrReviewRequestOnClosedPR) Error() string {
+	return "cannot request a re-review on a closed or merged PR"
+}
+
+func (err ErrReviewRequestOnClosedPR) Unwrap() error {
+	return util.ErrPermissionDenied
+}
+
 // ReviewType defines the sort of feedback a review gives
 type ReviewType int
 
@@ -618,9 +635,24 @@ func AddReviewRequest(ctx context.Context, issue *Issue, reviewer, doer *user_mo
 		return nil, err
 	}
 
-	// skip it when reviewer hase been request to review
-	if review != nil && review.Type == ReviewTypeRequest {
-		return nil, committer.Commit() // still commit the transaction, or committer.Close() will rollback it, even if it's a reused transaction.
+	if review != nil {
+		// skip it when reviewer hase been request to review
+		if review.Type == ReviewTypeRequest {
+			return nil, committer.Commit() // still commit the transaction, or committer.Close() will rollback it, even if it's a reused transaction.
+		}
+
+		if issue.IsClosed {
+			return nil, ErrReviewRequestOnClosedPR{}
+		}
+
+		if issue.IsPull {
+			if err := issue.LoadPullRequest(ctx); err != nil {
+				return nil, err
+			}
+			if issue.PullRequest.HasMerged {
+				return nil, ErrReviewRequestOnClosedPR{}
+			}
+		}
 	}
 
 	// if the reviewer is an official reviewer,