Support --username and environment variable to set username

[rolandwalker]

Description

• make --username an alias for --user at the CLI
• support an environment variable $MYSQL_USER which can be used in place of the CLI argument

Rationale: we support both --pass and --password. The distinction between --user and --username is hard to remember. All other connection coordinates support an environment variable.

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. High – MYSQL_USER now overrides explicit DSN intent and can disable DSN alias resolution
   In the new option definition, user is populated from env at parse time (envvar='MYSQL_USER'), so later DSN logic treats user as already set.
   • Alias resolution is skipped when user is truthy: mycli/main.py:2231
   • DSN username is ignored when user is already set: mycli/main.py:2260
   • Source of this behavior: mycli/main.py:1928
     This means MYSQL_USER can unexpectedly change connection target semantics (e.g., mycli prod no longer resolves prod DSN alias, or mysql://dsn_user@... silently uses env user instead).
     The new test currently codifies this regression: test/test_main.py:1173.

Actionable fix

• Treat MYSQL_USER as a fallback only after DSN/alias parsing (like: if user is None: user = os.environ.get("MYSQL_USER")), instead of binding it directly in Click.
• Update tests to assert precedence: CLI --user/--username > DSN username > MYSQL_USER and that DSN alias still resolves when MYSQL_USER is set.

Could not run tests locally because pytest/uv are not installed in this environment.