[Flight] Fix `hasReadable` flag in Node.js clients' debug channel (#35039)

For Edge Flight servers, that use Web Streams, we're defining the `debugChannel` option as: ``` debugChannel?: {readable?: ReadableStream, writable?: WritableStream, ...} ``` Whereas for Node.js Flight servers, that use Node.js Streams, we're defining it as: ``` debugChannel?: Readable | Writable | Duplex | WebSocket ``` For the Edge Flight clients, there is currently only one direction of the debug channel supported, so we define the option as: ``` debugChannel?: {readable?: ReadableStream, ...} ``` Consequently, for the Node.js Flight clients, we define the option as: ``` debugChannel?: Readable ``` The presence of a readable debug channel is passed to the Flight client internally via the `hasReadable` flag on the internal `debugChannel` option. For the Node.js clients, that flag was accidentally derived from the public option `debugChannel.readable`, which is conceptually incorrect, because `debugChannel` is a `Readable` stream, not an options object with a `readable` property. However, a `Readable` also has a `readable` property, which is a boolean that indicates whether the stream is in a readable state. This meant that the `hasReadable` flag was incidentally still set correctly. Regardless, this was confusing and unintentional, so we're now fixing it to always set `hasReadable` to `true` when a `debugChannel` is provided to the Node.js clients. We'll revisit this in case we ever add support for writable debug channels in Node.js (and Edge) clients.