fix(pytest_plugin): correct session_params docstring

why: Docstring said "Return new, temporary Session" but the fixture returns
dict[str, Any] — a factual error found by 2/9 brainstorm originals.
what:
- Change summary line to "Return default session creation parameters."

Author: tony

src/libtmux/pytest_plugin.py

@@ -152,7 +152,7 @@ def fin() -> None:
 
 @pytest.fixture
 def session_params() -> dict[str, t.Any]:
-    """Return new, temporary :class:`libtmux.Session`.
+    """Return default session creation parameters.
 
     >>> import pytest
     >>> from libtmux.session import Session