refactor(ocpp): harmonize post-response event listener pattern across stacks

refactor(ocpp): harmonize post-response event listener pattern across stacks

Move post-response logic from inline handler to event listeners:
- OCPP16 UpdateFirmware: fire-and-forget in handler → event listener
  matching OCPP20 UpdateFirmware pattern
- OCPP20 RequestStopTransaction: await in handler → event listener
  matching OCPP16 RemoteStopTransaction pattern

All commands with post-response behavior now use the same pattern:
handler validates and returns response, event listener performs
the async action after the response is sent to the CSMS.