share a generous test I/O timeout constant

The flaky-gate fixes kept hand-tuning magic seconds in each real-I/O
test. Pull them into one Flutter-free constant — ioTimeout (20s) in
test/helpers/timeouts.dart, importable by both the dart-test (pty) and
flutter-test suites — and route the real-external-wait timeouts through
it: PTY output (session + registry), and fs-watcher events (timeout +
poll ceiling). Tune in one place instead of scattering durations.

The ipc socket round-trip timeouts (2s) are left as-is — they haven't
flaked and local sockets respond in ms; they can adopt the constant
later if needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/helpers/timeouts.dart

@@ -0,0 +1,10 @@
+/// Shared test timeouts. Flutter-free on purpose, so both `dart test`
+/// (the pty / ipc suites) and `flutter test` files can import it.
+library;
+
+/// Generous timeout for waiting on real external I/O — PTY output, fs
+/// watcher events, socket round-trips. A working path responds in well
+/// under a second; this is pure headroom so transient scheduling latency
+/// doesn't flake the gate. A genuinely broken path still fails — just
+/// later. Tune here, once, rather than scattering magic seconds.
+const Duration ioTimeout = Duration(seconds: 20);