[hugosmoreira]

Fixes #723.

More() temporarily ignores SIGINT while an external pager runs, but currently resets it to SIG_DFL afterward and leaves it ignored if paging raises. Save the previous handler and restore it in finally, preserving the original exception.

Add regression tests for custom/default/ignored handlers, explicit and discovered pagers, startup/encoding/communication/wait failures, and the Display() caller. Noninteractive output and internal-pager routes have compatibility controls.

Validation:

• Six new test methods pass on Windows Python 3.13 and Linux Python 3.7, 3.12 and 3.14. The pre-fix run and negative controls produce 16 failed assertions.
• Linux Python 3.12: 279 tests pass both without and with IPython.
• Windows Python 3.13: 271 pass and eight fail in both variants; the same eight failures occur on unchanged master.
• CI lint, new-test lint, wheel/sdist builds and built-wheel regression tests pass on Windows and Linux. Broader lint findings and the Linux 3.12 type-check failures match unchanged master.
• Linux 3.7/3.14 integration checks run a real cat pager and deliver SIGINT afterward; the custom callback is retained. Unchanged master loses it.

macOS and the remaining hosted matrix were not run. Interactive detection is controlled in the pager checks; no manual terminal session is claimed.

#666 overlaps exception cleanup but still restores SIG_DFL. This change preserves the previous handler without changing command parsing or fallback behavior.