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">

(cherry picked from commit 242b331260925e604150346e61329097d5731e77)

services/pull/review_test.go

@@ -0,0 +1,48 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package pull_test
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	pull_service "code.gitea.io/gitea/services/pull"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDismissReview(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	pull := unittest.AssertExistsAndLoadBean(t, &issues_model.PullRequest{})
+	assert.NoError(t, pull.LoadIssue(db.DefaultContext))
+	issue := pull.Issue
+	assert.NoError(t, issue.LoadRepo(db.DefaultContext))
+	reviewer := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})
+	review, err := issues_model.CreateReview(db.DefaultContext, issues_model.CreateReviewOptions{
+		Issue:    issue,
+		Reviewer: reviewer,
+		Type:     issues_model.ReviewTypeReject,
+	})
+
+	assert.NoError(t, err)
+	issue.IsClosed = true
+	pull.HasMerged = false
+	assert.NoError(t, issues_model.UpdateIssueCols(db.DefaultContext, issue, "is_closed"))
+	assert.NoError(t, pull.UpdateCols(db.DefaultContext, "has_merged"))
+	_, err = pull_service.DismissReview(db.DefaultContext, review.ID, issue.RepoID, "", &user_model.User{}, false, false)
+	assert.Error(t, err)
+	assert.True(t, pull_service.IsErrDismissRequestOnClosedPR(err))
+
+	pull.HasMerged = true
+	pull.Issue.IsClosed = false
+	assert.NoError(t, issues_model.UpdateIssueCols(db.DefaultContext, issue, "is_closed"))
+	assert.NoError(t, pull.UpdateCols(db.DefaultContext, "has_merged"))
+	_, err = pull_service.DismissReview(db.DefaultContext, review.ID, issue.RepoID, "", &user_model.User{}, false, false)
+	assert.Error(t, err)
+	assert.True(t, pull_service.IsErrDismissRequestOnClosedPR(err))
+}