Move _MOCK_TEXTUAL_KEYS, _build_mock_textual, _install_mock_textual,
_restore_modules, _make_persona_state, _cleanup_tmpdir, and
_FakeCommandRouter out of tui_app_coverage_steps.py into a shared
_tui_mock_helpers.py module so both step files can import them without
duplication and the coverage steps file stays within the 500-line budget.
ISSUES CLOSED: #6361
