Add Python_ExecutionRequest and Python_ExecutionResponse to the
IpcChannel enum and replace the bare string literals used by
PythonService (main) and PyodideService (renderer) so the push-style
request/response channels are governed the same way as Python_Execute.
Wire values are normalized to the repository's 'namespace:action-dashed'
convention (python:execution-request / python:execution-response) since
both ends are updated in the same change.
