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/pty_size.dart

@@ -0,0 +1,15 @@
+/// Minimum dimension handed to any PTY backend.
+///
+/// A 1-column ConPTY makes the Windows conhost spin emitting CRLF forever
+/// (microsoft/terminal#19922), and a 0 in either axis is invalid on both
+/// platforms. Every backend clamps its spawn + resize through this, so a
+/// degenerate size from the UI — a pane measured at zero width during a
+/// transient layout pass — can never wedge a child. The floor (2) is below
+/// any real terminal, so the clamp is invisible in normal use.
+library;
+
+/// Smallest column/row count a PTY backend will accept.
+const int minPtyDimension = 2;
+
+/// Clamp a column or row count up to [minPtyDimension].
+int clampPtyDimension(int value) => value < minPtyDimension ? minPtyDimension : value;