Add permission check when creating PR (#31033) (#31720)

Backport #31033

user should be a collaborator of the base repo to create a PR

routers/web/repo/pull.go

@@ -1325,6 +1325,16 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 				return
 			}
 			ctx.JSONError(flashError)
+		} else if errors.Is(err, issues_model.ErrMustCollaborator) {
+			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
+				"Message": ctx.Tr("repo.pulls.push_rejected"),
+				"Summary": ctx.Tr("repo.pulls.new.must_collaborator"),
+			})
+			if err != nil {
+				ctx.ServerError("CompareAndPullRequest.HTMLString", err)
+				return
+			}
+			ctx.JSONError(flashError)
 		}
 		return
 	}