Fix UnicodeDecodeError when guessed charset doesn't hold for the whole file

[Oscar A Garcia (alexandregfonseca)]

Summary

• PlainTextConverter guesses the file's charset from only the first 4096 bytes of the stream (_markitdown.py). If that prefix happens to be pure ASCII but a multi-byte UTF-8 character appears later in the file, the charset is mis-detected as ascii, and the subsequent full-content .decode("ascii") raises UnicodeDecodeError, causing conversion to fail entirely.
• This is easy to hit in practice: running markitdown README.md on this very repo's README fails, because the README is ASCII for its first 4096 bytes and contains an em dash (—) shortly after.
• Fix: when decoding with the guessed charset fails, fall back to detecting the charset from the full file content instead of just the first 4096 bytes.

Test plan

• Added a regression test (test_plain_text_charset_guessed_from_prefix) reproducing the exact scenario: 4096 bytes of ASCII followed by a UTF-8 multi-byte character.
• pytest packages/markitdown/tests/test_module_misc.py passes (one unrelated pre-existing failure in test_speech_transcription due to ffprobe not being installed in my environment).
• Manually verified markitdown README.md now succeeds instead of raising FileConversionException.

[Oscar A Garcia (alexandregfonseca)]

@microsoft-github-policy-service agree

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