fix(test): restore the issue-2039 suite by completing its session-actions mock

The suite mocks `session-actions` by listing its exports one by one, and had
fallen behind `unarchiveSession`/`unarchiveSessions`. `session-ui-store` imports
both, so the file threw on import and ran zero tests — the draft auto-accept and
canonical-worktree-directory guarantees it covers were unprotected, and the
report looked almost like silence rather than a failure.

packages/ui/src/sync/__tests__/issue-2039.test.ts

@@ -251,6 +251,8 @@ mock.module("../session-actions", () => ({
   deleteSessions: mock(async () => ({ deletedIds: [], failedIds: [] })),
   archiveSession: mock(async () => true),
   archiveSessions: mock(async () => ({ archivedIds: [], failedIds: [] })),
+  unarchiveSession: mock(async () => true),
+  unarchiveSessions: mock(async () => ({ restoredIds: [], failedIds: [] })),
   updateSessionTitle: mock(async () => undefined),
   shareSession: mock(async () => undefined),
   unshareSession: mock(async () => undefined),