Update with merge commit feature not working

[NPorcu-BHS]

Maybe related to or caused by issue #8231

The feature is still working with v0.126.0 and stopped working somewhere between v0.126.0 and v0.128.0 and is still broken in the latest prerelease v0.129.2026022604

• Extension version: v0.129.2026022604 (pre-release), v0.128.0
• VSCode Version: 1.109.5 (system setup)
• OS: Windows 11
• GitHub Product (GitHub.com/GitHub Enterprise version x.x.x): GitHub Enterprise Server 3.17.7

Steps to Reproduce:

1. Checkout branch with open pull request
2. Base branch should contain new changes
3. Open pull request extension web view ("This branch is out-of-date with base branch." is visible)
4. Click "Update with merge commit" button
5. Loading bar appears for a blink of a second
6. Nothing happens

Logs after button click:
2026-02-27 11:03:57.234 [error] [GitHubRepository] Updating branch using GraphQL UpdatePullRequestBranch mutation failed: Invariant Violation: Invariant Violation: 9 (see https://github.com/apollographql/invariant-packages)
2026-02-27 11:03:59.584 [info] [FolderRepositoryManager+0] Using in-memory cached assignable users.
2026-02-27 11:03:59.821 [info] [Authentication] Creating hub for enterprise
2026-02-27 11:03:59.821 [info] [Authentication] Enterprise server authority [REDACTED GH ENTERPRISE URL]
2026-02-27 11:04:00.016 [info] [FolderRepositoryManager+0] Found GitHub remote for folder [REDACTED FOLDER STRUCTURE]
2026-02-27 11:04:00.017 [info] [FolderRepositoryManager+0] Using in-memory cached mentionable users.
2026-02-27 11:04:00.017 [info] [FolderRepositoryManager+0] Using in-memory cached assignable users.