Correct swagger annotations for enums, status codes, and notification state (#37030)

## ⚠️ BREAKING ⚠️

- delete reaction endpoints is changed to return 204 No Content rather
than 200 with no content.

## Summary

Add swagger:enum annotations and migrate all enum comments from the
deprecated comma-separated format to JSON arrays. Introduce
NotifySubjectStateType with open/closed/merged values. Fix delete
reaction endpoints to return 204 instead of 200.

routers/api/v1/repo/pull.go

@@ -898,6 +898,8 @@ func MergePullRequest(ctx *context.APIContext) {
 	// responses:
 	//   "200":
 	//     "$ref": "#/responses/empty"
+	//   "403":
+	//     "$ref": "#/responses/forbidden"
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 	//   "405":