Deprecate DSN environment variable in favor of MYSQL_DSN

[rolandwalker]

Description

Unlike similar environment variables, this didn't have the MYSQL_ prefix. We can deprecate pretty freely here since this was not documented.

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. Medium – Deprecation warning is emitted even when legacy DSN is not used
   In mycli/main.py:2347, the warning is printed whenever DSN exists in the environment, even if cli_args.dsn was already set via --dsn or MYSQL_DSN.
   Action: only emit the warning when fallback actually applies (move click.secho(...) inside if not cli_args.dsn:), and ideally only when DSN is non-empty.

2. Low – Missing tests for the new env-var migration path
   This PR changes option env binding and adds manual fallback, but there are no tests covering it in test/pytests/test_main.py.
   Action: add tests for:

   • MYSQL_DSN populates cli_args.dsn (happy path).
   • DSN fallback works when MYSQL_DSN is unset, with deprecation warning.
   • precedence when both are set (MYSQL_DSN should win), and warning behavior is explicit/intentional.

No direct security issue found in the diff.

I couldn’t run tests locally because pytest is not installed in this environment.