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>

routers/api/v1/api.go

@@ -1369,6 +1369,7 @@ func Routes() *web.Router {
 						m.Combo("/requested_reviewers", reqToken()).
 							Delete(bind(api.PullReviewRequestOptions{}), repo.DeleteReviewRequests).
 							Post(bind(api.PullReviewRequestOptions{}), repo.CreateReviewRequests)
+						m.Post("/comments/{id}/replies", reqToken(), mustNotBeArchived, bind(api.CreatePullReviewCommentReplyOptions{}), repo.CreatePullReviewCommentReply)
 					})
 					m.Get("/{base}/*", repo.GetPullRequestByBaseHead)
 				}, mustAllowPulls, reqRepoReader(unit.TypeCode), context.ReferencesGitRepo())