chore: update output schema for parse and extract_tables

[SeisSerenata]

Description

This PR modifies the output schema for the parse and extract_tables functions to consistently return markdown content as a list instead of a joined string. This change provides more flexibility for downstream processing while maintaining backward compatibility through list joining where needed.

Related Issue

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement

How Has This Been Tested?

• Updated all existing tests to handle the new list-based output format
• Tests have been modified to join the markdown list elements when comparing with ground truth
• All test cases pass with the new schema

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Additional Notes

Key changes:

1. Modified parse() and extract_tables() to return markdown as a list instead of joining it
2. Updated async_fetch() to maintain consistency with the new return format
3. Updated all test cases to handle the new list-based output format
4. Maintained backward compatibility by joining lists where needed for comparison

[lingjiekong]

LGTM.

[lingjiekong]

LGTM with minor comment

[lingjiekong]

nit: it might be better to check the result page to page.