refactor(ocpp): remove as-unknown-as double-casts from request/response dispatch...

refactor(ocpp): remove as-unknown-as double-casts from request/response dispatch (#2003)

Give each builder its precise input contract instead of laundering casts
through phantom generics or JsonType:

- buildRequestPayload drops its unsound <Request> generic and returns honest
  JsonType; each dispatch branch narrows with a single cast at a genuine
  JsonType boundary.
- Builder inputs (StatusNotificationOptions, OCPP20TransactionEventOptions,
  SignCertificateOptions) are JsonObject subtypes; StatusNotification build
  validates connectorStatus via the shared isOCPP20ConnectorStatus guard.
- Handler bridges (toRequestHandler/toResponseHandler) use a single cast that
  preserves the handler's async identity, fixing an unawaited-handler
  regression from a plain wrapper defeating isAsyncFunction.
- CAT-C string-to-enum narrowing becomes a type guard / typed Record.

Closes #1968