Add DISTINCT to value-position keywords

[rolandwalker]

Description

Major omission. Covered under the existing changelog.

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 .

[github-actions]

1. Low: Missing regression test for the new completion behavior
   • Change adds DISTINCT to pygments_value_position_nonfunction_keywords in mycli/sqlcompleter.py:839, but there is no test that validates SQLCompleter now actually offers DISTINCT in value-position/function-style completion contexts.
   • Existing DISTINCT coverage in test/test_completion_engine.py:178 only checks parser suggestion type (suggest_type), not final completion output.
   • Action: add a completer-level test (likely in test/test_smart_completion_public_schema_only.py or test/test_naive_completion.py) asserting DISTINCT appears in completions for a SELECT ... value-position scenario.

No correctness bug or security issue stood out in the one-line code change itself.

Note: I couldn’t run tests in this environment because pytest is not installed (pytest: command not found).