Fix per-database timezone datetime handling

[Hugo Moreira (hugosmoreira)]

Summary

• Adapt aware datetime parameters to the configured database connection time zone before binding them to SQL Server's time-zone-less datetime2 columns.
• Convert datetime SQL expressions from the connection time zone to the requested time zone instead of assuming the stored source is UTC.
• Add regressions covering parameter adaptation, exact lookups, ORM round trips, local-date lookups, and both SQL conversion helper paths.

Root cause

convert_datetimefield_value() interpreted time-zone-less values in connection.timezone, while adapt_datetimefield_value() always converted aware parameters to UTC. This made reads and writes asymmetric whenever DATABASES[alias]["TIME_ZONE"] was non-UTC.

The datetime SQL conversion helpers also added only the target offset. Once parameter binding honors the connection time zone, those helpers must convert from the connection offset to the target offset.

Fixes #371

Validation

• python manage.py test testapp.tests --noinput -v 1 — 214 tests completed successfully (3 skipped, 3 expected failures).
• Django 6.0.8 upstream timezones module — 84 tests completed successfully (3 skipped, 7 expected failures).
• Focused TestDateTimeField SQL Server tests — 4 passed.
• Negative controls confirmed the regressions fail with the original UTC binding, and that changing only parameter binding breaks upstream per-database time-zone conversion.
• python -m compileall -q mssql testapp
• git diff --check

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.