[DOC, TST] Match longer IPython continuation prompts

[ac1982]

Fixes #217.

The documented prompt regex accepts exactly three continuation dots. IPython's Sphinx directive uses more dots as the cell counter grows, so following the example drops continuation lines from copied code after cell 9.

Use \.{3,} in both the documented configuration and the documentation site's conf.py. Update the pattern table and live example to show multi-digit counters. The generic copying implementation and extension defaults are unchanged.

Added tests read the actual docs configuration, check it agrees with the documented snippet, and copy multiline inputs numbered 1, 10, 100 and 1000. The last three fail with the original configuration.

Validation:

• AVA with the CI's Node 12: 22 tests pass.
• Full Sphinx HTML build on Python 3.12 and all pre-commit hooks pass.
• Clicking the live example's copy button in Chromium preserves both continuation lines; loading the original generated copybutton.js against the same page loses the multi-digit continuation.
• System Node 24 cannot run the existing esm-based test harness, so validation used Node 12. No dependency changes are included.

Prepared and validated with Codex assistance.