fix(ocpp2): align MeterValues implementation with OCPP 2.0.1 spec (#1744)

fix(ocpp2): align MeterValues implementation with OCPP 2.0.1 spec (#1744)

* fix(ocpp2): use AlignedDataInterval for standalone MeterValues

Add getAlignedDataInterval() helper to OCPP20ServiceUtils that reads
AlignedDataCtrlr.Interval from the variable registry (default 900s).

Replace getTxUpdatedInterval() with getAlignedDataInterval() in the
broadcast channel handleMeterValues OCPP 2.0 branch. Standalone
MeterValuesRequest is non-transaction data per OCPP 2.0.1 spec and
should use the aligned data interval, not the tx-updated interval.

* fix(ocpp2): include meter value in TransactionEvent Started

Add buildTransactionBeginMeterValues() to OCPP20ServiceUtils following
the buildFinalMeterValues() pattern. Builds an OCPP20MeterValue with
Transaction.Begin context and Energy.Active.Import.Register measurand.

Wire it into both TransactionEvent(Started) call sites:
- OCPPServiceUtils.startTransactionOnConnector (ATG/broadcast channel)
- OCPP20IncomingRequestService RequestStartTransaction event listener

This aligns OCPP 2.0 with the OCPP 1.6 beginEndMeterValues behavior
per OCPP 2.0.1 spec SampledDataTxStartedMeasurands requirement.

* test(ocpp2): add tests for buildTransactionBeginMeterValues

Test Transaction.Begin context, energy register value, default to 0
when undefined, and empty array when energy is negative.

* refactor(ocpp2): address PR review comments

- Extract buildEnergyMeterValues private helper to eliminate DRY
  violation between buildTransactionBeginMeterValues and
  buildFinalMeterValues
- Add AlignedDataInterval to OCPP20RequiredVariableName enum replacing
  raw string literal in getAlignedDataInterval
- Clarify zero-energy test name to document that 0 Wh is a valid
  Transaction.Begin reading
- Add meterValue assertions to RequestStartTransaction test verifying
  Transaction.Begin context and Energy.Active.Import.Register measurand

* refactor(ocpp2): extract readVariableAsIntervalMs to eliminate DRY

getAlignedDataInterval and getTxUpdatedInterval were structurally
identical. Extract the shared variable-reading logic into a private
readVariableAsIntervalMs helper that accepts component name, variable
name, and default seconds. Both public methods become one-liner
delegates.

* refactor(ocpp2): extract terminateTransaction and resolveActiveTransaction

requestDeauthorizeTransaction and requestStopTransaction shared
identical transaction termination logic (build final meter values,
send TransactionEvent Ended, stop periodic, reset connector status).

Extract resolveActiveTransaction for the shared precondition check
and transactionId string resolution, and terminateTransaction for
the shared Ended event + cleanup workflow. Both public methods now
focus only on their unique behavior.

* fix(test): use enum constants instead of string literals in RequestStartTransaction test

Replace 'Transaction.Begin' and 'Energy.Active.Import.Register' string
literals with OCPP20ReadingContextEnumType.TRANSACTION_BEGIN and
OCPP20MeasurandEnumType.ENERGY_ACTIVE_IMPORT_REGISTER to match
codebase conventions.

* refactor(test): replace string literals with enum constants in OCPP 2.0 tests

Replace hardcoded string literals with their corresponding OCPP 2.0.1
enum values across test files for type safety and consistency:

- RequestStopTransaction: Transaction.End, Energy.Active.Import.Register
- SchemaValidation: Immediate, OnIdle, Heartbeat, BootNotification
- enforceMessageLimits: TooManyElements, TooLargeElement
- CertificateManager: Accepted, Failed, NotFound
- ChangeAvailability: UnknownEvse
- GetBaseReport: Accepted (SetVariableStatusEnumType)
- CertificateSigned: InternalError
- ServiceUtils-TransactionEvent: TransactionEvent command name

* fix(test): replace bogus mock return values with correctly typed empty responses

requestDeauthorizeTransaction mocks returned { status: 'Accepted' }
but OCPP20TransactionEventResponse has no status field. Replace with
properly typed empty response objects since the return value is not
asserted in these tests.

* refactor(ocpp2): align method names with OCPP 2.0.1 spec terminology

Rename methods to match OCPP 2.0.1 TransactionEvent terminology:
- buildTransactionBeginMeterValues -> buildTransactionStartedMeterValues
- buildFinalMeterValues -> buildTransactionEndedMeterValues
- beginMeterValues variable -> startedMeterValues
- finalMeterValues variable -> endedMeterValues

The spec uses Started/Updated/Ended for TransactionEvent types, not
Begin/Final.

* refactor(test): replace 'Operative' string literal with enum constants

Use OCPP20OperationalStatusEnumType.Operative in OCPP 2.0 test files
and the cross-version AvailabilityType.Operative in MessageChannelUtils
test to match ConnectorStatus.availability typed field.