Function prepare_grpc_conn 

pub fn prepare_grpc_conn(conn: Conn, codec_suffix: &str) -> Result<Conn, Conn>

Available on crate feature server only.

Validate request preflight (content-type, te:trailers) and set the gRPC response headers (content-type, grpc-accept-encoding). Returns the conn with those headers set, or an error-shaped conn if preflight failed.

Called from generated Handler::run after path matching has confirmed this request belongs to the service.