test_pane(test[set_title]): Add unicode test case for set_title

why: Docstring claimed "spaces and unicode" coverage but only tested ASCII.
what:
- Add unicode character (π) test case to test_set_title_special_characters

Author: tony

tests/test_pane.py

@@ -321,6 +321,9 @@ def test_set_title_special_characters(session: Session) -> None:
     pane.set_title("my pane title")
     assert pane.pane_title == "my pane title"
 
+    pane.set_title("my π pane")
+    assert pane.pane_title == "my π pane"
+
 
 def test_pane_context_manager(session: Session) -> None:
     """Test Pane context manager functionality."""