Allow /tee and /once to complete on any file extension

[rolandwalker]

Description

Whereas it makes sense for /edit and /source to complete only on files ending with .sql, the /tee and /once commands should be able to complete on any existing pathname, with any file extension.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .