fix: MsSQL timezone-safe timestamp normalization via AT TIME ZONE

[dtsong]

Summary

• Wrap MsSQL normalize_timestamp() with explicit UTC conversion
• Uses CAST(value AS DATETIMEOFFSET) AT TIME ZONE 'UTC' before formatting
• MsSQL cannot set session timezone, so this normalizes explicitly in SQL

Key Files

• data_diff/databases/mssql.py — normalize_timestamp() now converts to UTC

Test plan

• Unit tests pass (55/55)
• Ruff lint and format pass
• MsSQL timestamp normalization produces UTC-consistent output

Closes #30

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