Eqs 766 add html validation

[oms09]

Motivation and Context

After the investigation into validator gaps, HTML tags and entities was not being validated.

What has changed

• Added validate_html() to iterate through all translatable schema text.
• Implemented HTML tag validation using a stack-based approach.
• Ensures only allowed tags are used (p, strong, a, b, br, img), validates correct opening/closing tag structure, handles self-closing tags (br, img)
• Added validation for HTML entities Supports numeric (decimal and hexadecimal) and named entities, uses html.entities.html5 for validation
• Detects whitespace before br tags
• Ensures p tags appear at the start of a sentence (ignoring whitespace and wrapper characters)
• unit tests added

How to test?

make test
run validator and runner then run make validate-test-schemas to ensure there are no errors
check code

Links

https://officefornationalstatistics.atlassian.net/browse/EQS-766
Rhys Validation PR - #244

Screenshots (if appropriate)