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

Same root cause as 16cf9edf — checkout-git emits mainRepoRoot directly,
so the helper must initialize it to null or downstream isWorktree
derivation flips to true on undefined !== null.

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

@@ -59,6 +59,7 @@ function createCheckoutStatus(
   return {
     isGit: true,
     repoRoot: cwd,
+    mainRepoRoot: null,
     currentBranch: "main",
     isDirty: false,
     baseRef: "main",