refactor(ocpp2): consolidate dual-path request architecture into single path

refactor(ocpp2): consolidate dual-path request architecture into single path

Remove 8 unused dedicated request methods (requestFirmwareStatusNotification,
requestGet15118EVCertificate, requestGetCertificateStatus, etc.) that bypassed
buildRequestPayload via direct sendMessage calls. All production callers
already used requestHandler exclusively, making these methods dead code.

Move SignCertificate CSR generation logic into buildRequestPayload, making
it the authoritative payload construction layer — symmetric with OCPP 1.6.
This also adds isRequestCommandSupported check and AJV schema validation
to the SignCertificate flow that the dedicated method bypassed.

Refactor 6 test files to test through requestHandler (production path)
instead of the removed dedicated methods.