fix(tui): handle Windows PTY stdin and detached WS frames (#41953)

Two narrow Windows desktop fixes:

1. tools/process_registry.py — PTY stdin writes are now platform-aware.
   pywinpty (Windows) expects str; ptyprocess (POSIX) expects bytes.
   Previously bytes was unconditionally passed, producing a TypeError on
   Windows ("'bytes' object cannot be converted to 'PyString'").

2. tui_gateway/server.py + ws.py — Detached WebSocket sessions now park on
   a _DropTransport sink instead of _stdio_transport. In the desktop the
   gateway runs in-process and stdout is captured by Electron into
   desktop.log, so falling back to stdio leaked raw JSON-RPC frames into
   the desktop log after WS disconnects. Orphan-reap semantics are
   preserved via _ws_session_is_orphaned.

Verified on a Windows desktop install:
- pywinpty 2.0.15 rejects bytes / accepts str — reproduced exactly
- Focused suite green (write_stdin × 2, write_json_drops_detached_ws_frames,
  ws_orphan_reap × 2)
- All 6 CI test shards green, e2e green, nix (ubuntu/macos) green

Salvage commit (21be7ca) fixes the new test referencing an undefined
_ThreadUnsafeStdout — uses the existing _ChunkyStdout helper.

tests/test_tui_gateway_server.py

@@ -152,6 +152,21 @@ def test_write_json_returns_false_on_broken_pipe(monkeypatch):
     assert server.write_json({"ok": True}) is False
 
 
+def test_write_json_drops_detached_ws_frames(monkeypatch):
+    out = _ChunkyStdout()
+    monkeypatch.setattr(server, "_real_stdout", out)
+    server._sessions["detached-sid"] = {"transport": server._detached_ws_transport}
+    try:
+        assert server.write_json({
+            "jsonrpc": "2.0",
+            "method": "event",
+            "params": {"session_id": "detached-sid", "type": "message.delta"},
+        }) is False
+        assert out.parts == []
+    finally:
+        server._sessions.pop("detached-sid", None)
+
+
 def test_tui_verbose_tool_details_fail_closed_when_redaction_fails(monkeypatch):
     redact_module = types.ModuleType("agent.redact")
 
@@ -933,7 +948,7 @@ def test_ws_orphan_reap_closes_worker_when_session_stays_detached(monkeypatch):
             closed["worker"] = True
 
     server._sessions["orphan-sid"] = _session(
-        transport=server._stdio_transport,
+        transport=server._detached_ws_transport,
         slash_worker=_FakeWorker(),
         running=False,
     )
@@ -992,11 +1007,15 @@ def test_ws_orphan_reap_spares_reattached_session(monkeypatch):
     assert server._ws_session_is_orphaned(reattached) is False
 
     # Mid-turn sessions are also spared even if detached.
-    mid_turn = _session(transport=server._stdio_transport, running=True)
+    mid_turn = _session(transport=server._detached_ws_transport, running=True)
     assert server._ws_session_is_orphaned(mid_turn) is False
 
     # Already finalized sessions are spared (idempotency).
-    done = _session(transport=server._stdio_transport, running=False, _finalized=True)
+    done = _session(
+        transport=server._detached_ws_transport,
+        running=False,
+        _finalized=True,
+    )
     assert server._ws_session_is_orphaned(done) is False
 
 

tests/tools/test_process_registry.py

@@ -63,6 +63,44 @@ def _wait_until(predicate, timeout: float = 5.0, interval: float = 0.05) -> bool
     return False
 
 
+def test_write_stdin_uses_str_for_windows_pty(monkeypatch, registry):
+    """pywinpty expects str input; bytes raises a PyString conversion error."""
+    written = []
+
+    class _FakePty:
+        def write(self, value):
+            written.append(value)
+
+    session = _make_session(sid="pty-win")
+    session._pty = _FakePty()
+    registry._running[session.id] = session
+    monkeypatch.setattr("tools.process_registry._IS_WINDOWS", True)
+
+    result = registry.write_stdin(session.id, "hello\n")
+
+    assert result == {"status": "ok", "bytes_written": 6}
+    assert written == ["hello\n"]
+    assert isinstance(written[0], str)
+
+
+def test_write_stdin_uses_bytes_for_posix_pty(monkeypatch, registry):
+    written = []
+
+    class _FakePty:
+        def write(self, value):
+            written.append(value)
+
+    session = _make_session(sid="pty-posix")
+    session._pty = _FakePty()
+    registry._running[session.id] = session
+    monkeypatch.setattr("tools.process_registry._IS_WINDOWS", False)
+
+    result = registry.write_stdin(session.id, "hello\n")
+
+    assert result == {"status": "ok", "bytes_written": 6}
+    assert written == [b"hello\n"]
+
+
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