Complete filenames on more special commands

[rolandwalker]

Description

Although filename completion has a number of limitations currently, we should still offer it on each special command which accepts a filename, not just source.

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]

Findings

1. Case-sensitivity regression for \edit completion (low)

• In suggest_special(), the new filename-command check uses cmd.lower().
• \edit is registered as case-sensitive (\edit/\e, not \EDIT/\E), but completion now treats uppercase variants as valid and suggests filenames for invalid commands.
• Action: check cmd directly for case-sensitive commands (or split case-sensitive vs case-insensitive branches).

2. Missing test coverage for newly added commands (low)

• The PR adds filename completion paths for \e, \edit, \o, \once, and tee, but tests still only cover source/\. in:
  • test/test_completion_engine.py
  • test/test_smart_completion_public_schema_only.py
• Action: extend existing parametrized tests to include each newly supported command, including -o forms (tee -o , \once -o ).

Security

• No new security issues identified in this diff.