FIX Reject multi-character and empty punctuation_list entries in InsertPunctuationConverter

[Shivam Sharma (ShivamSharma43)]

Description

Fixes #3292.

InsertPunctuationConverter validated punctuation_list with char in string.punctuation, which is a substring test. Runs that happen to be contiguous in string.punctuation, such as "()" or "<=>", were accepted and inserted as-is, the empty string was accepted and inserted nothing, and an empty list passed validation and then failed with IndexError: Cannot choose from an empty sequence.

_is_valid_punctuation now requires a non-empty list whose entries are each a single character from string.punctuation, so all of these raise the documented ValueError: Invalid punctuations: .... Lists that were already valid, and the default list, behave exactly as before. The convert_async docstring now lists this ValueError.

Tests and Documentation

• New parametrized test_punctuation_list_rejects_substrings_of_string_punctuation covers ["()"], ["<=>"], ["{|}"], [""], [",", "!?"] and []. On main, 5 of the 6 cases fail; "!?" is not contiguous in string.punctuation, so it was already rejected and stays as a regression case. With this change, all of them pass.
• pytest tests/unit/converter/test_insert_punctuation_converter.py: 23 passed. pytest tests/unit/converter: 2732 passed, 34 skipped.
• ruff check, ruff format and ty check pass on the changed files. git diff --check is clean.
• No documentation changes. JupyText not run, since the converter docs do not pass invalid punctuation lists.

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