fix: avoid UnicodeDecodeError when non-ASCII bytes follow the first 4k

[André Cia (AndreCia)]

Problem

Charset detection in _markitdown.py inspects only the first 4096 bytes of a stream. A text file whose non-ASCII bytes appear past that window is labeled ascii, and decoding the full content then fails:

FileConversionException: File conversion failed after 1 attempts:
 - PlainTextConverter threw UnicodeDecodeError with message:
   'ascii' codec can't decode byte 0xe2 in position 5030: ordinal not in range(128)

Minimal reproduction: 5000 ASCII characters followed by café. Detection over the first 4k returns ascii; detection over the full content returns utf_8.

This affects .md, .txt, .csv and other text formats large enough to exceed the detection window. It is easy to hit with real documents: an English-language file whose first accented character, em dash, or curly quote happens to sit past the 4k mark converts fine until the day that character moves earlier or later.

Fix

_markitdown.py — when detection returns ASCII and the stream continues past the inspected page, the guess is widened to UTF-8. ASCII is a strict subset of UTF-8, so nothing that decoded before stops decoding.

The size condition is the important part. A stream that ends within the 4096-byte page was fully inspected, so ASCII still holds and is kept. Without that condition, guess_stream_info() would start reporting utf-8 for small, purely ASCII files, which breaks the test.json and test_notebook.ipynb vectors in test_module_vectors.py. Those vectors are correct as written, so the fix is scoped to the case where detection was actually partial.

converter_utils/charset.py (new) — decode_text() centralizes decoding: it tries the supplied charset and, on UnicodeDecodeError, re-detects over every byte. This covers misdetected non-UTF-8 charsets (cp1252, latin-1), where widening to UTF-8 is not enough. It also handles from_bytes(...).best() returning None, which previously produced the literal string "None" as the document's content.

_plain_text_converter.py and _csv_converter.py — use the helper.

Tests

5 new cases in tests/test_charset_detection.py: UTF-8 past the detection window, the same for CSV, recovery from a wrong charset, honoring a valid explicit charset, and decoding with no charset supplied.

Full suite: 341 passed, 4 skipped. black clean.

One test note for macOS: the CLI tests invoke python via subprocess, and macOS ships only python3 on PATH, so they need the venv on PATH (PATH="$PWD/.venv/bin:$PATH") or 50 tests fail for environmental reasons unrelated to any change.

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