fix(repo): surface unrelated histories on Sync Fork (#39258)

Sync Fork already maps merge conflicts to a JSON error. Unrelated
histories still went through `ServerError`, so the UI showed a 500 HTML
snippet instead of the same user-facing message PR merge already uses
(`repo.pulls.unrelated_histories`).

The API path returned 500 for the same git error; PR merge returns 409.
Match that.

Fixes #36772

AI assistance was used to locate the handler gap and draft the mapping.
I reviewed and take responsibility for the change.

Signed-off-by: Zhaoqi Xu <lzy00419@outlook.com>

templates/swagger/v1-swagger.generated.json

@@ -14997,6 +14997,9 @@
           },
           "404": {
             "$ref": "#/responses/notFound"
+          },
+          "409": {
+            "$ref": "#/responses/error"
           }
         }
       }