TestWatchFailed is flaky

## Flaky Test

### Which jobs are failing
```text
keyspace_group_manager_test.go:181:
Error Trace: /home/runner/work/pd/pd/tests/integrations/mcs/tso/keyspace_group_manager_test.go:181
/home/runner/work/pd/pd/tests/integrations/mcs/tso/keyspace_group_manager_test.go:197
Error: Not equal:
expected: 0x94
actual : 0x0
Test: TestTSOKeyspaceGroupManagerSuite/TestWatchFailed
```

### CI link
- https://github.com/tikv/pd/actions/runs/24888890229/job/72875770462
- https://github.com/tikv/pd/actions/runs/24954128915/job/73069314473
- https://github.com/tikv/pd/actions/runs/24981006703/job/73142960241

### Reason for failure (if possible)

### Anything else
- Test: `TestTSOKeyspaceGroupManagerSuite/TestWatchFailed`
- Package: `mcs/tso`
- Seen in PRs: #10620, #10621, #10622

Start with tests/integrations/mcs/tso/keyspace_group_manager_test.go, especially lines 181 and 197, and run TestTSOKeyspaceGroupManagerSuite/TestWatchFailed. Compare the expected 0x94 with the actual 0x0 and the linked CI failures; done means the test no longer flakes and passes reliably.

Written by the indexing model from the issue text.