fix: use configured timezone for MySQL session time_zone

[Br1an67]

Description

When use_tz=True, the MySQL client sets the session time_zone using timezone.now().utcoffset(). Since timezone.now() returns a UTC-aware datetime, its utcoffset() is always timedelta(0), causing the session to always be set to +0:00 regardless of the configured timezone (e.g. Asia/Shanghai).

This PR replaces timezone.now() with get_default_timezone() to compute the correct UTC offset from the user-configured timezone.

Motivation and Context

Fixes #2114

How Has This Been Tested?

• Added test_mysql_session_timezone_uses_configured_tz in tests/backends/test_connection_params.py
• The test configures USE_TZ=True and TIMEZONE=Asia/Shanghai, then verifies the SET time_zone call uses +8:00
• Existing tests continue to pass

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

---

_{Comparing Br1an67:fix/mysql-session-timezone (57d0cfe) with develop (03703a7)¹}

Footnotes

1. No successful run was found on main (d914c70) during the generation of this report, so develop (03703a7) was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[abondar]

@Br1an67 Hi! Check linter here please