test(charging-station): add message buffering, reservation tests and OCPP spec reference

test(charging-station): add message buffering, reservation tests and OCPP spec reference

- Add Message Buffering tests (8 tests): buffer/flush cycle, offline queueing,
  flush ordering, edge cases with rapid connect/disconnect
- Add Reservation Management tests (5+ tests): add/remove reservations,
  query by reservationId/idTag/connectorId, connector 0 restrictions,
  multiple reservations handling
- Create OCPPSpecRequirements.md with OCPP specification references for all
  test domains (1.6, 2.0.1, 2.1) covering boot notification, heartbeat,
  WebSocket protocol, transactions, and connector states
- Fix lint errors in ChargingStationTestUtils.ts (type safety improvements)

Test count: 136 tests across 10 domains
Coverage: Lifecycle, Connector/EVSE, Boot Notification, Configuration,
         WebSocket, Transactions, Heartbeat/Ping, Error Recovery,
         Message Buffering, Reservations