fix: ImageConverter gracefully handles LLM API failures (fixes #1942)

[hanhan761]

Summary

ImageConverter._get_llm_description() crashes the entire conversion pipeline when the LLM API call fails (auth error, rate limit, network timeout). The API call at _image_converter.py:137 had no error handling.

This fix wraps the API call in a try/except block, matching the pattern already used by PptxConverter (lines 119-129). On failure, it returns None and the converter gracefully skips the LLM description while still returning extracted metadata.

Issue

Fixes #1942

Verification

• 6/7 existing tests pass (1 pre-existing failure due to missing outlook dependency)
• Conversion without LLM client works unchanged