test(inspect): extract accessibility tests into check_a11y_test.go

Mechanical follow-up to keep every check test file under 1000 LOC. Moves the
ARIA / landmarks / advanced-a11y tests verbatim out of check_test.go into a
focused same-package file. No behavior, API, or exported-symbol changes:
- check_test.go (700 LOC)
- check_a11y_test.go (441 LOC)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Patel230