harden the ConPTY backend (Linux-verifiable pre-VM work)

Pre-Windows-VM hardening — the parts validatable on Linux, leaving the
unrunnable FFI (Job Object, T-424) for the VM session:

- Clamp PTY cols/rows to >= 2 in both backends' spawn + resize (new
  pty_size.dart). A 1-column ConPTY makes conhost spin emitting CRLF
  (microsoft/terminal#19922); 0 is invalid on both platforms.
- ci/test.sh: --timeout 60s on the dart-test pty line (matches the flutter
  lines) so a wedged ConPTY reader fails fast instead of hanging the run.
- Make windows_pty.dart's pure helpers public + testable off-Windows:
  quoteArg (MSVCRT quoting), composeEnvironmentBlock, and resolveExecutable
  (now takes an injectable existence probe). New windows_pty_args_test.dart
  + pty_size_test.dart give 15 cross-platform assertions over the trickiest
  Windows logic the on-Windows smoke suite can't reach off-platform.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/pty/native_pty.dart

@@ -25,6 +25,7 @@ import 'dart:typed_data';
 import 'package:ffi/ffi.dart';
 
 import 'errors.dart';
+import 'pty_size.dart';
 import '../ipc/errno_mapping.dart' show PosixErrno;
 import 'ffi/libc.dart' as libc;
 import 'pty_session.dart';
@@ -310,8 +311,8 @@ class NativePty implements PtySession {
 
     // ---- Set initial winsize on the master ---------------------------
     final ws = calloc<_Winsize>()
-      ..ref.wsRow = rows
-      ..ref.wsCol = columns;
+      ..ref.wsRow = clampPtyDimension(rows)
+      ..ref.wsCol = clampPtyDimension(columns);
     _ioctl(masterFd, _kTiocsWinsz, ws);
     calloc.free(ws);
 
@@ -429,8 +430,8 @@ class NativePty implements PtySession {
   void resize({required int cols, required int rows}) {
     if (_dead) return;
     final ws = calloc<_Winsize>()
-      ..ref.wsRow = rows
-      ..ref.wsCol = cols;
+      ..ref.wsRow = clampPtyDimension(rows)
+      ..ref.wsCol = clampPtyDimension(cols);
     final rc = _ioctl(_fd, _kTiocsWinsz, ws);
     calloc.free(ws);
     if (rc < 0 && libc.errno == PosixErrno.ebadf) {