feat(pty): expose terminal modes in SSHPtyConfig

[wonkyungup]

Summary

Adds an optional modes: Map<int, int>? field to SSHPtyConfig and wires it through SSHClient.shell() and
SSHClient.execute() via the existing terminalModes parameter of sendPtyReq — which previously had no
public API to populate.

Motivation

The SSH pty-req message (RFC 4254 §8) carries an "encoded terminal modes" field — opcode/value pairs that
let the client request specific termios settings at PTY creation time. Without exposing this at the API
level, callers have no way to control PTY behavior before the first byte hits the line discipline.

The concrete use case that motivated this: a terminal client that injects shell integration (OSC 7 cwd
reporting, prompt hooks) right after connection. Doing client.shell() then writing stty -echo; <setup>; stty echo\n doesn't work — the PTY echoes input character-by-character until the newline arrives, by which
time the entire setup line has already been streamed to the user's screen. The only race-free fix is to
disable ECHO before the PTY is created, which requires sending mode 53 (ECHO) with value 0 in the
initial pty-req.

Other common uses: {51: 0, 53: 0} for raw-mode TUIs, custom VINTR/VERASE mappings, etc.

Changes

• SSHPtyConfig: new final Map<int, int>? modes field and constructor parameter.
• SSHPtyConfig.encodeModes(): encodes the map into the byte string format defined by RFC 4254 §8 —
  opcode(1) + value(4 BE) per entry, terminated by TTY_OP_END (0x00). Returns Uint8List(0) when modes is
  null or empty.
• SSHClient.shell() and SSHClient.execute(): pass pty.encodeModes() as terminalModes to sendPtyReq.

Backward compatibility

Fully backward compatible. modes is optional and defaults to null; encodeModes() returns an empty byte
string in that case, which is identical to the previous behavior (sendPtyReq already defaulted
terminalModes to Uint8List(0)).

Example

final session = await client.shell(
  pty: const SSHPtyConfig(
    type: 'xterm-256color',
    width: 120,
    height: 40,
    modes: {53: 0}, // ECHO off at PTY creation
  ),
);

Spec

RFC 4254 §8 — https://datatracker.ietf.org/doc/html/rfc4254#section-8