test(server): default mainRepoRoot to null in createCheckoutStatus helper

de428582 made workspace-git-service propagate checkoutStatus.mainRepoRoot
directly. The test helper never set the field, so the runtime snapshot
emitted undefined where tests expected null.

packages/server/src/server/workspace-git-service.test.ts

@@ -89,6 +89,7 @@ function createCheckoutStatus(
   return {
     isGit: true,
     repoRoot: cwd,
+    mainRepoRoot: null,
     currentBranch: "main",
     isDirty: false,
     baseRef: "main",