isTextualContentType

Whether contentType identifies a body worth treating as text. Shared with the OkHttp/Ktor adapter modules so a response (or request) body is never buffered into memory on the host's calling thread only to be discarded here as binary -- adapters call this before reading any bytes to decide whether reading is worth doing at all. NetworkCaptureFactory itself still runs the same check (plus a UTF-8 decodability fallback for a body an adapter did read despite an absent content-type) once bytes are actually in hand.