test(monitor): add comprehensive test suite for engine and checkers
55 tests covering state machine transitions, heartbeat handling, push deadline checks, group aggregation, history recording, probe aggregation, log management, state management, and concurrency safety. Checker tests cover HTTP (via httptest), port (via net.Listen), isCodeAccepted ranges, and siteTimeout defaults. Ping and DNS checkers skipped (need ICMP privileges and DNS server). Coverage: 64.2% overall, 100% on handleStatusChange, triggerAlert, checkPush, recordCheck, and AggregateStatus.
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