test(ocpp): add call chain integration tests for both OCPP stacks

test(ocpp): add call chain integration tests for both OCPP stacks

Verify the single-path contract: requestHandler(minimal params) →
buildRequestPayload(constructs) → sendMessage(complete payload).

OCPP 2.0: StatusNotification, TransactionEvent (with default resolution
for triggerReason, transactionId, connectorId from evse), Heartbeat,
and rawPayload bypass.

OCPP 1.6: StatusNotification (errorCode added by builder),
StartTransaction (meterStart/timestamp enrichment), StopTransaction
(meterStop/timestamp enrichment), Heartbeat.

Remove deleted TransactionContextFixtures from TEST_STYLE_GUIDE.