Add tests for PlainTextConverter

[daere1]

Summary

PlainTextConverter (packages/markitdown/src/markitdown/converters/_plain_text_converter.py) had no dedicated unit tests. This adds coverage for:

• Accepted file extensions (.txt, .text, .md, .markdown, .json, .jsonl)
• Accepted mimetype prefixes (text/, application/json, application/markdown)
• accepts() short-circuiting to True whenever stream_info.charset is set, regardless of extension/mimetype
• Decoding via a declared charset vs. charset_normalizer auto-detection when no charset is declared

Testing

• python -m pytest packages/markitdown/tests/test_plain_text_converter.py -v — 12 passed
• python -m pytest packages/markitdown/tests -q — 900 passed, 5 skipped, 13 pre-existing failures unrelated to this change (Windows console-encoding issues in test_cli_vectors.py subprocess tests, and a test_speech_transcription failure from a missing ffprobe/network dependency in this environment)

This is my first contribution to this project 🙂

🤖 Generated with Claude Code

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