Improve test coverage of the curses module

[serhiy-storchaka]

Feature or enhancement

Lib/test/test_curses.py references almost every curses window method and module function, but many references only check that a call does not raise, and some of the API is not exercised at all. Areas worth deepening:

• curses.panel — the stacking semantics (only the userptr lifecycle is covered).
• curses.textpad — Textbox behaviour (currently driven through a MagicMock) and rectangle().
• Window output/input — addstr/insstr/insch and the pad operations, verified by reading the cells back.
• Error handling — out-of-range coordinates and bad argument types.
• Entry points with no coverage — curses.wrapper(), window.insch(), curses.panel.update_panels() and curses.has_key().

Linked PRs

• gh-151693: Add curses tests for panels, textpad, and window behavior #151694

Linked PRs

• [3.15] gh-151693: Add curses tests for panels, textpad, and window behavior (GH-151694) #151719
• [3.14] gh-151693: Add curses tests for panels, textpad, and window behavior (GH-151694) #151720
• [3.13] gh-151693: Add curses tests for panels, textpad, and window behavior (GH-151694) #151721
• gh-151693: Make the curses tests portable to other curses implementations #151729
• [3.13] gh-151693: Make the curses tests portable to other curses implementations (GH-151729) #151730
• [3.14] gh-151693: Make the curses tests portable to other curses implementations (GH-151729) #151731