[CI/Build] Do not abort test collection when pexpect is missing

[GuoCheng24]

CONTRIBUTING.md says to run tests with pytest. On a clean checkout that aborts before collecting anything:

tests/test_tui_e2e.py:18: in <module>
    import pexpect
E   ModuleNotFoundError: No module named 'pexpect'
!!!!!! Interrupted: 1 error during collection !!!!!!

pexpect is declared nowhere — not in pyproject.toml dependencies or extras, not in install.sh, not in uv.lock. Neither is pytest itself. The file was clearly meant to be skippable: line 23 is pytestmark = pytest.mark.skipif(shutil.which("script") is None, ...). But that mark is evaluated after the module import, so the import takes the whole suite down instead of skipping this one file — 56 test files for one optional dependency.

The change. pexpect = pytest.importorskip("pexpect"), which skips this module and leaves the rest of the suite collecting, plus a dev extra declaring the test-only dependencies.

Verified both directions with pexpect blocked at import:

before:  ERROR tests/test_tui_e2e.py — Interrupted: 1 error during collection
after:   no tests collected in 0.03s   (this file skipped, collection continues)

AI assistance: partial — the reproduction, the fix and the before/after verification were done by running pytest with the import blocked on a clean venv, then confirming the rest of the suite still collects.