fix(html): preserve case-insensitive checkbox types

[LI (ktz03)]

HTML checkboxes with type="CHECKBOX" or mixed-case values currently lose their checked/unchecked markers during conversion. For example, <p><input type="CHECKBOX" checked>Task</p> becomes Task rather than [x] Task.

Match checkbox types using ASCII case-insensitive comparison, as required for HTML enumerated attributes. Keep missing, empty, unrelated, whitespace-padded, and non-ASCII type values ignored. Add public MarkItDown.convert_stream regressions for both checked states and those controls.

Validation on Windows with Python 3.14.3, markdownify 1.2.3, Beautiful Soup 4.15.0, and Magika 0.6.2:

• The new regressions fail 6 cases on main and pass all 15 with the fix.
• HTML and library infrastructure suites: 218 passed, 2 provider-backed tests skipped.
• pre-commit run --all-files: passed (Black 23.7.0).