russh: Client-side channel-scoped Handler callbacks fire for channel IDs the client never opened

### Summary CVE-2026-68930 was fixed by adding `Session::is_established_channel()` in `russh/src/server/encrypted.rs`, which gates every channel-scoped SERVER-side message (CHANNEL_REQUEST, CHANNEL_DATA, CHANNEL_EOF, CHANNEL_CLOSE, CHANNEL_WINDOW_ADJUST, CHANNEL_EXTENDED_DATA) on `enc.channels.get(&channel).is_some_and(|c| c.confirmed)` before invoking any `Handler` callback. The identical validation was never added to the CLIENT side (`russh/src/client/encrypted.rs`), which processes channel-scoped messages sent by the SSH SERVER once the client has authenticated. ### Details In `client_read_authenticated` (`russh/src/client/encrypted.rs`, ~lines 431-757), for CHANNEL_DATA, CHANNEL_EXTENDED_DATA, CHANNEL_EOF, CHANNEL_CLOSE, CHANNEL_OPEN_FAILURE, CHANNEL_SUCCESS, CHANNEL_FAILURE, and the CHANNEL_REQUEST sub-types exit-status/exit-signal/xon-xoff, the code only optionally forwards the event to the internal per-channel mpsc sender via `if let Some(chan) = self.channels.get(&channel_num) { ... }` (a no-op if the channel is unknown), but then **unconditionally** calls the corresponding public `Handler` trait method (`client.data(...)`, `client.exit_status(...)`, `client.channel_close(...)`, `client.channel_success(...)`, etc.) regardless of whether `channel_num` corresponds to any channel the client ever opened or that was ever confirmed. Only CHANNEL_OPEN_CONFIRMATION (closes the connection with `Error::Inconsistent` if unknown) and CHANNEL_WINDOW_ADJUST (returns early with `Ok(