Deprecate $MYSQL_UNIX_PORT environment variable

[rolandwalker]

Description

Deprecate $MYSQL_UNIX_PORT environment variable in favor of $MYSQL_UNIX_SOCKET. We can do this pretty freely since the feature was not documented.

Motivation: a Unix Domain Socket is not a TCP/IP Port, though both are available on Unix and Unix-likes. The variable name was very confusing.

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. Missing regression tests for env var migration path (low)
   • Change introduces new behavior at main.py:1924 and fallback/deprecation logic at main.py:2215, but this PR adds no coverage in test/test_main.py.
   • Add targeted CLI tests for:
     • MYSQL_UNIX_SOCKET is honored.
     • MYSQL_UNIX_PORT still works (with warning).
     • precedence when both are set (MYSQL_UNIX_SOCKET wins).
     • explicit --socket overriding env vars.

No correctness or security bugs stood out in the patch itself.

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