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

[johntmyers]

Summary

Supersedes #573. Reimplemented cleanly on a fresh branch with review feedback addressed.

• Replace flat pty_master/input_sender/pty_request fields in SshHandler with HashMap<ChannelId, ChannelState> so each channel tracks its own PTY resources independently
• Fix set_winsize to pass &winsize to ioctl, correcting undefined behavior on aarch64
• Add warn! logging for unknown channels across all handlers (window_change_request, data, channel_eof) for debuggability
• Add channel_close handler to clean up per-channel state and prevent FD leaks

Related Issue

Resolves #543

Changes

File	Change
crates/openshell-sandbox/src/ssh.rs	Introduce ChannelState struct; migrate per-channel fields into HashMap<ChannelId, ChannelState>; update all handlers to look up channel state by ID; fix ioctl UB; add warn! logging for unknown channels

Improvements over #573

1. cargo fmt clean — no formatting violations (CI-blocking issue in fix(sandbox): track PTY state per SSH channel to fix terminal resize #573)
2. Consistent unknown-channel logging — window_change_request, data, and channel_eof now all log warn! on unknown channels, matching the error-returning pattern in pty_request/subsystem_request/start_shell
3. channel_close documented — comment explains it subsumes channel_eof for cleanup

Testing

• set_winsize_applies_to_correct_pty — verifies two PTYs can be independently resized using real PTYs via openpty
• channel_state_independent_input_senders — verifies data routing and EOF isolation between channels
• All existing tests pass (cargo test -p openshell-sandbox)

Checklist

• Tests added for new functionality
• mise run pre-commit passes (license check failure is pre-existing in architecture/plans/)
• No unrelated changes