Ext Proc Report Grpc Error On Client Creation Failure

Fixed a bug in the :ref:ext_proc <envoy_v3_api_msg_extensions.filters.http.ext_proc.v3.ExternalProcessor> HTTP filter where a failure to create the gRPC client for the external processor caused Filter::openStream() to short-circuit to IgnoreError regardless of :ref:failure_mode_allow <envoy_v3_api_field_extensions.filters.http.ext_proc.v3.ExternalProcessor.failure_mode_allow>. The underlying ProcessorClientImpl::start() returned nullptr without invoking onGrpcError/onGrpcClose, so the consuming filter could not distinguish a client-creation failure from a successful no-op. The client now reports the failure via onGrpcError with status INTERNAL so the consuming filter applies the configured failure_mode_allow semantic. This behavior can be temporarily reverted by setting envoy.reloadable_features.ext_proc_report_client_creation_error to false.