[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.