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.

modules/structs/pull_review.go

@@ -8,6 +8,8 @@ import (
 )
 
 // ReviewStateType review state type
+//
+// swagger:enum ReviewStateType
 type ReviewStateType string
 
 const (
@@ -21,10 +23,11 @@ const (
 	ReviewStateRequestChanges ReviewStateType = "REQUEST_CHANGES"
 	// ReviewStateRequestReview review is requested from user
 	ReviewStateRequestReview ReviewStateType = "REQUEST_REVIEW"
-	// ReviewStateUnknown state of pr is unknown
-	ReviewStateUnknown ReviewStateType = ""
 )
 
+// ReviewStateUnknown is an internal sentinel for unknown review state, not a valid API value.
+const ReviewStateUnknown = ""
+
 // PullReview represents a pull request review
 type PullReview struct {
 	ID                int64           `json:"id"`