fix(sandbox): track PTY state per SSH channel to fix terminal resize

[cluster2600]

Summary

Resolves #543 — terminal resize (window_change_request) was broken when multiple SSH channels were open simultaneously.

• Replace the flat pty_master/input_sender/pty_request fields in SshHandler with a HashMap<ChannelId, ChannelState>, ensuring each channel tracks its own PTY resources independently
• Fix set_winsize to pass a reference (&winsize) to ioctl, correcting undefined behaviour on aarch64
• Add error logging when set_winsize fails, rather than silently discarding the result

Root Cause

The previous implementation stored a single pty_master file descriptor at the handler level. When a client opened multiple channels (e.g. parallel shells, shell + SFTP), window_change_request would resize whichever PTY was stored last — not the one belonging to the requesting channel.

graph TD
    subgraph "Before (broken)"
        A[Channel 0 — resize 80×24] --> B[SshHandler.pty_master]
        C[Channel 1 — resize 120×50] --> B
        B --> D["ioctl(TIOCSWINSZ) on last-stored FD"]
    end

    subgraph "After (fixed)"
        E[Channel 0 — resize 80×24] --> F["channels[0].pty_master"]
        G[Channel 1 — resize 120×50] --> H["channels[1].pty_master"]
        F --> I["ioctl(TIOCSWINSZ) on channel 0 FD"]
        H --> J["ioctl(TIOCSWINSZ) on channel 1 FD"]
    end

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Changes

File	Change
crates/openshell-sandbox/src/ssh.rs	Introduce ChannelState struct; migrate all per-channel fields into HashMap<ChannelId, ChannelState>; update pty_request_request, window_change_request, data, channel_eof, subsystem_request, and spawn_shell_or_exec to look up channel state by ID

Test Plan

• cargo test -p openshell-sandbox — 319 passed (1 pre-existing failure in drop_privileges, unrelated)
• New test: set_winsize_applies_to_correct_pty — verifies two PTYs can be resized independently
• New test: channel_state_independent_input_senders — verifies data routing and EOF isolation between channels

sequenceDiagram
    participant Client
    participant SshHandler
    participant Channel0
    participant Channel1

    Client->>SshHandler: pty_request(channel=0, 80×24)
    SshHandler->>Channel0: store PTY master + request

    Client->>SshHandler: pty_request(channel=1, 120×50)
    SshHandler->>Channel1: store PTY master + request

    Client->>SshHandler: window_change(channel=0, 132×43)
    SshHandler->>Channel0: ioctl(TIOCSWINSZ, 132×43)
    Note over Channel1: unaffected

    Client->>SshHandler: channel_eof(channel=0)
    SshHandler->>Channel0: drop input_sender
    Note over Channel1: still functional

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