run the PTY tests serially instead of retrying them

The pty-tagged tests spawn real PTYs and flaked when dart test ran them in
parallel (fd contention) — papered over with retry: 2. Run that pass with
--concurrency=1 and drop the retries: serialization is the correct fix for
resource-bound tests. Verified stable across repeated runs.

T-193.

Co-Authored-By: Claude <noreply@anthropic.com>

ci/test.sh

@@ -24,8 +24,11 @@ flutter analyze
 echo "==> dart format (whole tree)"
 dart format --set-exit-if-changed .
 
-echo "==> dart test (pty — unreliable under the flutter test runner)"
-dart test --tags pty test/pty/session_test.dart test/panes/registry_test.dart
+echo "==> dart test (pty — unreliable under the flutter test runner; serial)"
+# --concurrency=1: these spawn real PTYs and compete for fds when run in
+# parallel, which flaked them (registry/session). Serialize — the proper fix
+# for resource-bound tests, vs. the old per-test `retry:` band-aid. (T-193)
+dart test --concurrency=1 --tags pty test/pty/session_test.dart test/panes/registry_test.dart
 
 if [[ "$coverage" == 1 ]]; then
   echo "==> flutter test --coverage (gate; unit + widget + golden + a11y)"

test/panes/registry_test.dart

@@ -43,7 +43,7 @@ void main() {
       expect(evt.data['id'], pane.id);
     });
 
-    test('output events base64-encode the child bytes', tags: ['pty'], retry: 2, () async {
+    test('output events base64-encode the child bytes', tags: ['pty'], () async {
       // Subscribe to the sink stream BEFORE spawn so we don't miss
       // any pane.output events that arrive between spawn and listen.
       final buf = StringBuffer();

test/pty/session_test.dart

@@ -26,7 +26,7 @@ void main() {
   if (!Platform.isLinux && !Platform.isMacOS) return;
 
   group('NativePty', () {
-    test('spawns shell -c echo and reads output', tags: ['pty'], retry: 2, () async {
+    test('spawns shell -c echo and reads output', tags: ['pty'], () async {
       final s = NativePty.start(
         executable: '/bin/sh',
         arguments: ['-c', 'echo hello-pty'],
@@ -44,7 +44,7 @@ void main() {
       expect(got, contains('hello-pty'));
     });
 
-    test('write sends keystrokes to child', tags: ['pty'], retry: 2, () async {
+    test('write sends keystrokes to child', tags: ['pty'], () async {
       final s = NativePty.start(
         executable: '/bin/sh',
         arguments: [],
@@ -79,7 +79,7 @@ void main() {
       expect(result, contains('write-test-ok'));
     });
 
-    test('close kills child and closes output', tags: ['pty'], retry: 2, () async {
+    test('close kills child and closes output', tags: ['pty'], () async {
       final s = NativePty.start(
         executable: '/bin/sh',
         arguments: [],
@@ -103,7 +103,7 @@ void main() {
       expect(s.isClosed, isTrue);
     });
 
-    test('bare command name resolves via the PATH env var', tags: ['pty'], retry: 2, () async {
+    test('bare command name resolves via the PATH env var', tags: ['pty'], () async {
       // 'sh' is a bare command; without resolution, execve would fail.
       final s = NativePty.start(
         executable: 'sh',