Ability to complete partial enum values

[rolandwalker]

Description

Since the implementation of enum completions, the list of candidates was only presented at the start of the possible enum values. The user could select from the menu of candidates by tab or arrow keys, but not narrow down the list of candidate by typing text. And when a the user had set a positive min_completion_trigger value in ~/.myclirc, no enum value candidates would be offered after reaching the trigger.

With this change, completions are offered on the possible values of an enum similar to other completions, with partially-typed values respected. Candidates are case-ignoring, and completion is case- correcting.

Limitation: can fail when there is a space within the enum value, due to the way word_before_cursor works. This seems like a more general problem, and not a blocker here.

Incidentally add some unrelated tests in test_sqlcompleter.py.

xref #1427

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 .