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>

test/pty/pty_size_test.dart

@@ -0,0 +1,23 @@
+/// Unit tests for the shared PTY-dimension clamp (`pty_size.dart`). Both
+/// backends route spawn + resize through it so a degenerate (0/1) terminal
+/// size can never reach a child — see microsoft/terminal#19922.
+library;
+
+import 'package:clide/src/pty/pty_size.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('clampPtyDimension', () {
+    test('raises sub-minimum values to the floor', () {
+      expect(clampPtyDimension(0), minPtyDimension);
+      expect(clampPtyDimension(1), minPtyDimension);
+      expect(clampPtyDimension(-5), minPtyDimension);
+    });
+
+    test('passes through values at or above the floor', () {
+      expect(clampPtyDimension(minPtyDimension), minPtyDimension);
+      expect(clampPtyDimension(80), 80);
+      expect(clampPtyDimension(24), 24);
+    });
+  });
+}