Add test for case-insensitive HTML tag handling

[coderabbitai]

Description

Add a test using the existing html_table_uppercase fixture to ensure convert_html_tables handles uppercase tags case-insensitively.

Context

This improvement was suggested during PR review to ensure robust handling of HTML tables regardless of tag case.

Acceptance Criteria

• Add test using html_table_uppercase fixture
• Verify convert_html_tables processes uppercase HTML tags correctly
• Ensure case-insensitive handling is documented through tests

References

• PR: Add tests for HTML table conversion #24
• Comment: Add tests for HTML table conversion #24 (comment)