enable_fts(): accept a one-shot iterator for columns

[syahra712]

Fixes #902.

enable_fts() declares columns: Iterable[str] but walks it up to five times (the CREATE VIRTUAL TABLE statement, populate_fts(), and three times for the triggers). With a generator or iter([...]) only the first pass sees any columns, so the FTS table gets created and then populate_fts() fails with near ")": syntax error, leaving an empty _fts table behind.

This turns columns into a list once at the top of the method.

The new test passes iter(["text", "country"]) with and without create_triggers; both cases fail on main and pass here. tests/test_fts.py is green and black --check is clean.

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📚 Documentation preview 📚: https://sqlite-utils--903.org.readthedocs.build/en/903/