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>

templates/swagger/v1_json.tmpl

@@ -14470,6 +14470,75 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/pulls/{index}/comments/{id}/replies": {
+      "post": {
+        "consumes": [
+          "application/json"
+        ],
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "repository"
+        ],
+        "summary": "Reply to a pull request review comment",
+        "operationId": "repoCreatePullReviewCommentReply",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "owner of the repo",
+            "name": "owner",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "name of the repo",
+            "name": "repo",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "integer",
+            "format": "int64",
+            "description": "index of the pull request",
+            "name": "index",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "integer",
+            "format": "int64",
+            "description": "id of the review comment to reply to",
+            "name": "id",
+            "in": "path",
+            "required": true
+          },
+          {
+            "name": "body",
+            "in": "body",
+            "required": true,
+            "schema": {
+              "$ref": "#/definitions/CreatePullReviewCommentReplyOptions"
+            }
+          }
+        ],
+        "responses": {
+          "201": {
+            "$ref": "#/responses/PullReviewComment"
+          },
+          "400": {
+            "$ref": "#/responses/validationError"
+          },
+          "404": {
+            "$ref": "#/responses/notFound"
+          },
+          "422": {
+            "$ref": "#/responses/validationError"
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/pulls/{index}/commits": {
       "get": {
         "produces": [
@@ -24088,6 +24157,17 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "CreatePullReviewCommentReplyOptions": {
+      "description": "CreatePullReviewCommentReplyOptions are options to reply to a pull request review comment",
+      "type": "object",
+      "properties": {
+        "body": {
+          "type": "string",
+          "x-go-name": "Body"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "CreatePullReviewOptions": {
       "description": "CreatePullReviewOptions are options to create a pull request review",
       "type": "object",