Add API endpoint to reply to pull request review comments (#36683)

Adds a dedicated endpoint for replying to pull request review comments,

```
POST /repos/{owner}/{repo}/pulls/{index}/comments/{id}/replies
{ "body": "..." }
```

The reply is threaded under the same review as the parent comment.

Ref: https://gitea.com/gitea/gitea-mcp/issues/129
Fixes: https://github.com/go-gitea/gitea/issues/37419
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/structs/pull_review.go

@@ -94,6 +94,11 @@ type CreatePullReviewComment struct {
 	NewLineNum int64 `json:"new_position"`
 }
 
+// CreatePullReviewCommentReplyOptions are options to reply to a pull request review comment
+type CreatePullReviewCommentReplyOptions struct {
+	Body string `json:"body" binding:"Required"`
+}
+
 // SubmitPullReviewOptions are options to submit a pending pull request review
 type SubmitPullReviewOptions struct {
 	Event ReviewStateType `json:"event"`