fix: let Document Intelligence use SDK default API version

[he-yufeng]

Fixes #1904.

Summary

• stop forcing the older Document Intelligence preview API version by default
• only pass api_version to DocumentIntelligenceClient when the caller supplies one
• add focused constructor tests that do not call Azure

Verified

• .venv\Scripts\python.exe -m pytest packages\markitdown\tests\test_docintel_html.py -q
• .venv\Scripts\python.exe -m py_compile packages\markitdown\src\markitdown\converters_doc_intel_converter.py packages\markitdown\tests\test_docintel_html.py
• git diff --check