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>

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();