feat: add GIF support to ImageConverter

[UditDewan]

Summary

Fixes #1103

ImageConverter rejected GIF files with UnsupportedFormatException, even though exiftool (used for metadata extraction) and multimodal LLM APIs both handle GIFs. As noted by @afourney in the issue, there's no reason not to support them.

Changes

• Add image/gif to ACCEPTED_MIME_TYPE_PREFIXES and .gif to ACCEPTED_FILE_EXTENSIONS in _image_converter.py. The existing metadata and LLM-captioning code paths work unchanged: mimetypes.guess_type already resolves .gif correctly, so the data URI sent to the LLM gets the proper image/gif content type.
• Add tests/test_files/test.gif (a 43-byte 1x1 GIF) and test_markitdown_gif_image in test_module_misc.py, which verifies a GIF converts without raising and that the image is sent to the LLM client as a data:image/gif;base64, URI.

Testing

• New test passes, along with the full test_module_misc.py and test_module_vectors.py local-file suites.