feat: Add default PR branch update style setting (#37410)

Adds repository-level settings for pull request branch updates so admins
can choose the default update method and disable merge or rebase
updates.

<img width="1025" height="158"
src="https://github.com/user-attachments/assets/d030973b-0ddd-4035-b04f-145c445084d7"
/>

---------

Co-authored-by: OpenAI Codex (GPT-5) <codex@openai.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1_openapi3_json.tmpl

@@ -5619,6 +5619,11 @@
             "type": "boolean",
             "x-go-name": "AllowMerge"
           },
+          "allow_merge_update": {
+            "description": "either `true` to allow updating pull request branch by merge, or `false` to prevent it.",
+            "type": "boolean",
+            "x-go-name": "AllowMergeUpdate"
+          },
           "allow_rebase": {
             "description": "either `true` to allow rebase-merging pull requests, or `false` to prevent rebase-merging.",
             "type": "boolean",
@@ -5669,6 +5674,11 @@
             "type": "string",
             "x-go-name": "DefaultMergeStyle"
           },
+          "default_update_style": {
+            "description": "set to an update style to be used by this repository: \"merge\" or \"rebase\".",
+            "type": "string",
+            "x-go-name": "DefaultUpdateStyle"
+          },
           "description": {
             "description": "a short description of the repository.",
             "type": "string",
@@ -9115,6 +9125,10 @@
             "type": "boolean",
             "x-go-name": "AllowMerge"
           },
+          "allow_merge_update": {
+            "type": "boolean",
+            "x-go-name": "AllowMergeUpdate"
+          },
           "allow_rebase": {
             "type": "boolean",
             "x-go-name": "AllowRebase"
@@ -9184,6 +9198,10 @@
             "type": "string",
             "x-go-name": "DefaultTargetBranch"
           },
+          "default_update_style": {
+            "type": "string",
+            "x-go-name": "DefaultUpdateStyle"
+          },
           "description": {
             "type": "string",
             "x-go-name": "Description"