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

[dtsong]

Problem

MsSQL cannot set a session timezone (mssql.py:85 raises NotImplementedError). When comparing MsSQL timestamps against a UTC-normalized database (e.g., PostgreSQL), phantom diffs appear for any timestamps where the MsSQL server timezone differs from UTC.

Solution

MsSQL supports AT TIME ZONE 'UTC' syntax. Wrap timestamp normalization in normalize_timestamp() to explicitly convert to UTC, bypassing the need for a session-level setting.

Key Files

• data_diff/databases/mssql.py:85 (set_timezone_to_utc)
• data_diff/databases/mssql.py (normalize_timestamp)

Acceptance Criteria

• normalize_timestamp() wraps values with AT TIME ZONE 'UTC' conversion
• Cross-database diff between MsSQL and PostgreSQL produces correct results regardless of MsSQL server timezone
• Unit test covers the SQL generation