fix(cli,tui): escape and validate SSH session response fields

[johntmyers]

Summary

• The CLI and TUI build an SSH ProxyCommand string by interpolating fields from CreateSshSessionResponse (sandbox_id, token, and the assembled gateway URL) into a shell command. These three fields were not passed through shell_escape, even though exe_command and gateway_name in the same format strings were.
• OpenSSH invokes ProxyCommand through /bin/sh -c, so a hostile or compromised gateway could inject shell metacharacters ($(...), backticks, ;, |) and execute arbitrary commands on the developer's workstation under their user privileges.
• This PR centralizes the fix by extracting a build_proxy_command helper in openshell-core::forward that shell-escapes every interpolated argument, and adds a validate_ssh_session_response boundary validator that rejects malformed responses at the gRPC trust boundary — belt-and-suspenders with the escape.

Related Issue

OS-100

Changes

• crates/openshell-core/src/forward.rs
  • New build_proxy_command(exe, gateway_url, sandbox_id, token, gateway_name) — one source of truth for the ProxyCommand format string; every argument goes through shell_escape.
  • New validate_ssh_session_response(&CreateSshSessionResponse) + SshSessionResponseError enum. Enforces conservative charsets per field: sandbox_id [A-Za-z0-9._-]{1,128}, token URL-safe ASCII up to 4096 bytes, gateway_host IP/bracketed-IPv6/DNS hostname ≤253 bytes, gateway_scheme exactly http|https, gateway_port in 1..=65535, connect_path RFC 3986 path charset with mandatory leading /, optional host_key_fingerprint [A-Za-z0-9:+/=-] only.
  • 13 new unit tests covering realistic responses and per-field adversarial inputs.
• crates/openshell-cli/src/ssh.rs
  • ssh_session_config calls validate_ssh_session_response immediately after response.into_inner(), then uses build_proxy_command.
  • render_ssh_config shell-escapes gateway and name (defense-in-depth — validate_gateway_name already gates gateway).
• crates/openshell-tui/src/lib.rs
  • All three ProxyCommand sites (shell connect, sandbox exec, port-forward reconciliation) validate the response and use build_proxy_command; validation failures surface via app.status_text / tracing::warn!.
• proto/openshell.proto
  • Documents the charset contract on CreateSshSessionResponse fields. Servers must uphold; clients reject.

Testing

• cargo test -p openshell-core — 110 tests pass, including 13 new tests for the validator and build_proxy_command.
• cargo test -p openshell-cli --lib — 78 tests pass.
• cargo test -p openshell-tui --lib — 12 tests pass.
• mise run pre-commit — passes (lint, format, license headers, rust tests).
• Regression check: build_proxy_command_escapes_shell_metacharacters builds the command with attacker-controlled values (x$(touch /tmp/pwn)x, tok`id`) and asserts no $, backtick, |, ;, &, or newline appears outside single-quoted regions.
• Regression check: validate_ssh_session_response_rejects_* covers sandbox_id="a;b", token="$(id)", gateway_host="evil; cmd", gateway_scheme="javascript", gateway_port=0/70000, connect_path="/x + backtick + id + backtick + y"/"no-leading-slash"/"/a b"/"/a\nb".

Checklist

• Conventional Commits format
• No secrets or credentials in diff
• Scoped to the issue at hand (no unrelated refactors)
• Documentation updated where applicable (proto comment)

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.