The format_header() function was not calling format_inline_content()
to process inline formatting like **bold**, *italic*, and `code`
within headers. This caused raw markdown markers to appear in output.
Added 4 tests to verify the fix:
- test_bold_inside_header
- test_italic_inside_header
- test_code_inside_header
- test_mixed_formatting_inside_header
39918cf281