Expose content_version for optimistic locking on issue and PR edits (#37035)

- Add `content_version` field to Issue and PullRequest API responses
- Accept optional `content_version` in `PATCH
/repos/{owner}/{repo}/issues/{index}` and `PATCH
/repos/{owner}/{repo}/pulls/{index}` — returns 409 Conflict when stale,
succeeds silently when omitted (backward compatible)
- Pre-check `content_version` before any mutations to prevent partial
writes (e.g. title updated but body rejected)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1_json.tmpl

@@ -10362,6 +10362,7 @@
         }
       },
       "patch": {
+        "description": "Pass `content_version` to enable optimistic locking on body edits.\nIf the version doesn't match the current value, the request fails with 409 Conflict.\n",
         "consumes": [
           "application/json"
         ],
@@ -24766,6 +24767,11 @@
           "type": "string",
           "x-go-name": "Body"
         },
+        "content_version": {
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "ContentVersion"
+        },
         "due_date": {
           "type": "string",
           "format": "date-time",
@@ -24938,6 +24944,11 @@
           "type": "string",
           "x-go-name": "Body"
         },
+        "content_version": {
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "ContentVersion"
+        },
         "due_date": {
           "type": "string",
           "format": "date-time",
@@ -26223,6 +26234,11 @@
           "format": "int64",
           "x-go-name": "Comments"
         },
+        "content_version": {
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "ContentVersion"
+        },
         "created_at": {
           "type": "string",
           "format": "date-time",
@@ -27725,6 +27741,11 @@
           "format": "int64",
           "x-go-name": "Comments"
         },
+        "content_version": {
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "ContentVersion"
+        },
         "created_at": {
           "type": "string",
           "format": "date-time",