Assert checkpoint-ID validity via the production validator in e2e

AssertCheckpointIDFormat claimed to assert a valid checkpoint ID but matched against CheckpointPattern, which is now intentionally a loose extraction regex — it accepts ULID-shaped strings production rejects (e.g. a timestamp overflow starting 8-Z). Call checkpointid.Validate instead so the test rejects exactly what production rejects. Removes the now-unused checkpointIDPattern var and regexp import.