Add support for UYAP UDF files

[efekurucay]

Summary

This PR adds native support for UYAP UDF files in MarkItDown.

It introduces a built-in UdfConverter that:

• accepts .udf files directly
• detects UDF archives from zip-like streams by checking for content.xml and a <template> root
• parses rune-based text offsets from UDF XML
• converts supported content directly to Markdown without adding new runtime dependencies

Supported output in this initial version

• paragraphs
• bold / italic / underline
• numbered and bulleted lists
• basic tables

For now, embedded images are represented as [embedded image omitted] rather than causing conversion to fail.

Tests

This PR adds:

• focused unit tests for UDF detection, rune-based offsets, list behavior, empty paragraphs, nested table flattening, and image placeholder spacing
• a .udf fixture integrated into the existing vector-based test suite

Notes

This keeps the initial implementation intentionally conservative and does not yet attempt higher-fidelity formatting such as color, font size, or alignment semantics.

[efekurucay]

@microsoft-github-policy-service agree