tests(refactor[plugin_system]) Use Pane.display_message wrapper

why: Six tests in the plugin-system suite were querying tmux for
session/window names via the same `session.cmd("display-message",
"-p", "'#S'")` escape hatch that production just migrated away
from in `cli/load.py:_reattach`. Keeping the tests on the typed
wrapper that production now uses keeps the call shape consistent
across the codebase and removes the last `cmd("display-message",
...)` site from the repo.

what:
- `tests/cli/test_cli.py::test_reattach_plugins`: replace the
  `cmd()` call with `active_pane.display_message("'#S'",
  get_text=True)` so the post-reattach assertion uses the typed
  wrapper.
- `tests/workspace/test_builder.py`: migrate four
  `test_plugin_system_*` checks (`before_workspace_builder`,
  `on_window_create`, `after_window_finished`, and the
  multi-window variant covering both `before_script` and
  `after_window_finished`) onto `active_pane.display_message()`.
  The multi-window variant hoists the active_pane lookup once and
  reuses it for both the `'#S'` and `'#W'` queries.
- Each call site adds a narrowing assert on `active_pane`
  (libtmux types it as `Pane | None`) so mypy can prove the call
  is well-typed. Mirrors the production assert added in
  `_reattach()`.

No behavior change: tmux's `display-message` defaults to the
session's active pane when no `-t` is passed, so the underlying
tmux invocations are byte-for-byte identical to the prior
`session.cmd(...)` calls.

Author: tony

tests/cli/test_cli.py

@@ -170,6 +170,8 @@ def test_reattach_plugins(
         _reattach(builder)
 
     assert builder.session is not None
-    proc = builder.session.cmd("display-message", "-p", "'#S'")
+    active_pane = builder.session.active_pane
+    assert active_pane is not None
+    lines = active_pane.display_message("'#S'", get_text=True)
 
-    assert proc.stdout[0] == "'plugin_test_r'"
+    assert lines[0] == "'plugin_test_r'"

tests/workspace/test_builder.py

@@ -824,8 +824,10 @@ def test_plugin_system_before_workspace_builder(
 
     builder.build(session=session)
 
-    proc = session.cmd("display-message", "-p", "'#S'")
-    assert proc.stdout[0] == "'plugin_test_bwb'"
+    active_pane = session.active_pane
+    assert active_pane is not None
+    lines = active_pane.display_message("'#S'", get_text=True)
+    assert lines[0] == "'plugin_test_bwb'"
 
 
 def test_plugin_system_on_window_create(
@@ -847,8 +849,10 @@ def test_plugin_system_on_window_create(
 
     builder.build(session=session)
 
-    proc = session.cmd("display-message", "-p", "'#W'")
-    assert proc.stdout[0] == "'plugin_test_owc'"
+    active_pane = session.active_pane
+    assert active_pane is not None
+    lines = active_pane.display_message("'#W'", get_text=True)
+    assert lines[0] == "'plugin_test_owc'"
 
 
 def test_plugin_system_after_window_finished(
@@ -870,8 +874,10 @@ def test_plugin_system_after_window_finished(
 
     builder.build(session=session)
 
-    proc = session.cmd("display-message", "-p", "'#W'")
-    assert proc.stdout[0] == "'plugin_test_awf'"
+    active_pane = session.active_pane
+    assert active_pane is not None
+    lines = active_pane.display_message("'#W'", get_text=True)
+    assert lines[0] == "'plugin_test_awf'"
 
 
 def test_plugin_system_on_window_create_multiple_windows(
@@ -946,15 +952,18 @@ def test_plugin_system_multiple_plugins(
 
     builder.build(session=session)
 
+    active_pane = session.active_pane
+    assert active_pane is not None
+
     # Drop through to the before_script plugin hook
-    proc = session.cmd("display-message", "-p", "'#S'")
-    assert proc.stdout[0] == "'plugin_test_bwb'"
+    lines = active_pane.display_message("'#S'", get_text=True)
+    assert lines[0] == "'plugin_test_bwb'"
 
     # Drop through to the after_window_finished. This won't succeed
     # unless on_window_create succeeds because of how the test plugin
     # override methods are currently written
-    proc = session.cmd("display-message", "-p", "'#W'")
-    assert proc.stdout[0] == "'mp_test_awf'"
+    lines = active_pane.display_message("'#W'", get_text=True)
+    assert lines[0] == "'mp_test_awf'"
 
 
 def test_load_configs_same_session(