fix(sync): improve metrics recording and connection failure detection

Changes:
- Fix connection attempt metrics: record success/failure based on actual
  connection result instead of pre-emptively recording failure
- Add health tracker integration on connection failure: call
  record_failure() and record_health_state() in error path
- Add connection verification in relay_connection.rs: wait 500ms after
  connect() then verify is_connected() to detect silent failures
- Add configurable disconnect check interval via
  NGIT_SYNC_DISCONNECT_CHECK_INTERVAL_SECS env var
- Update TestRelay with fast test settings: startup_delay=0, jitter=0,
  disconnect_check_interval=1s
- Add debug output to metrics tests for investigation

Note: Tests may still fail due to 5-second base backoff in health tracker.
A follow-up task will add NGIT_SYNC_BASE_BACKOFF_SECS config parameter
to allow faster test cycles.

Related: metrics-wiring-plan.md Tasks 1 & 2

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