Improve distinguishing between DSNs and databases

[rolandwalker]

Description

We accept DSNs, both as literals and as aliases, as arguments to --database, as well as to --dsn. This might have been a mistake. The code is complex to support all cases, including the positional case, with selective overrides.

In short, while there are many edge cases, the user must be able to run

mycli --database alias

for backward compatibility with this feature. However, the user should also be able to override the database when connecting to a DSN, so

mycli alias --database overridden

should also be accepted.

However, note that the existing heuristic is that in the case of combination with flags which specify the coordinates of the server

mycli alias --user username

we make a different assumption: alias must refer to a database name, not a DSN. That heuristic may not be ideal, but is left in place for backward compatibility. We might consider changing it. The heuristic might require instead that all relevant coordinates be specified, instead of just one.

We also clarify here the CLI argument names dbname and database internally, to database (corresponding to the option name) and positional_database (since this is the positional value).

Finally, when an explicit --dsn is used, the user definitely wants a DSN, not a database name.

Incidentally, remove an outdated comment.

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 .