FIX: validate BinaryConverter bit width per converted word

[Umerkhan Golandaz (UmerkhanGolandaz)]

Description

Fixes #2590.

BinaryConverter.validate_input inspected the entire prompt, but WordLevelConverter.convert_async validates before applying the word selection strategy (word_level_converter.py:114 runs ahead of the selection at :118). A character in an unselected word therefore failed the whole conversion, even though that word is passed through unencoded and cannot overflow bits_per_char:

BinaryConverter(
    bits_per_char=BinaryConverter.BitsPerChar.BITS_16,
    word_selection_strategy=WordIndexSelectionStrategy(indices=[0]),
).convert_async(prompt="hello 👋")
# ValueError: bits_per_char=16 is too small for the characters in the prompt. Minimum required bits: 17.

Only hello is ever encoded here. The emoji is passed through as-is.

Changes

• Validate the bit width against each word that is actually converted, in convert_word_async, instead of against the whole prompt.
• Rename the helper to _validate_word, matching the style guide's rule that internal validation helpers are private (.github/instructions/style-guide.instructions.md).
• Add two regression tests: the error still raises when the offending word is selected, and conversion succeeds when it is not.

The ValueError message is unchanged. The default AllWordsSelectionStrategy path behaves identically, since whole-prompt and per-word validation agree when every word is converted — only the selective path changes.

Alternative considered

The same bug could be fixed generically in WordLevelConverter.convert_async by calling validate_input after word selection, which would cover any future subclass. I kept the change local to BinaryConverter because it is the only subclass that currently overrides validate_input, and that avoids changing the base class hook's contract. Happy to switch to the generic fix if you prefer it.

Tests

• pytest tests/unit/converter/ — 1440 passed, 34 skipped, including the 2 new tests
• ruff format --check and ruff check clean on both changed files

[Umerkhan Golandaz (UmerkhanGolandaz)]

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