test_server(fix[isolation]): add socket_name to test_tmux_bin_custom_path

why: Server created without socket_name queries the ambient tmux server,
breaking the isolation pattern used by all other tests in the file.
what:
- Add socket_name="test_tmux_bin" to the Server constructor

Author: tony

tests/test_server.py

@@ -426,7 +426,7 @@ def test_tmux_bin_custom_path(caplog: pytest.LogCaptureFixture) -> None:
     """Custom tmux_bin path is used for commands."""
     tmux_path = shutil.which("tmux")
     assert tmux_path is not None
-    s = Server(tmux_bin=tmux_path)
+    s = Server(socket_name="test_tmux_bin", tmux_bin=tmux_path)
     assert s.tmux_bin == tmux_path
     with caplog.at_level(logging.DEBUG, logger="libtmux.common"):
         s.cmd("list-sessions")