Preserve draft branch/worktree when update options are undefined

- Keep existing `branch` and `worktreePath` values when `setProjectDraftThreadId` receives `undefined`
- Add a regression test covering the undefined-update case
39fd14b73eJulius Marminge committed on 3/2/2026, 4:21:28 AMparentbd43372
2 files changedLine totals unavailable