fix: MsSQL timezone-safe timestamp normalization via AT TIME ZONE (#51)

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

Wrap MsSQL normalize_timestamp() with explicit UTC conversion using
CAST(value AS DATETIMEOFFSET) AT TIME ZONE 'UTC'. MsSQL cannot set
a session timezone, so we normalize explicitly in the SQL to ensure
consistent cross-database timestamp comparisons.

Closes #30

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: only apply UTC conversion for TimestampTZ columns in MsSQL

CAST(datetime AS DATETIMEOFFSET) assigns +00:00 unconditionally, not
the server's local timezone. UTC conversion only helps for datetimeoffset
columns (TimestampTZ). For timezone-naive datetime/datetime2 columns,
the conversion is a no-op — values are used as-is since the source
timezone is unknown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: dtsong

data_diff/databases/mssql.py

@@ -138,6 +138,13 @@ def constant_values(self, rows) -> str:
         return f"VALUES {values}"
 
     def normalize_timestamp(self, value: str, coltype: TemporalType) -> str:
+        # For timezone-aware columns (datetimeoffset), convert to UTC explicitly
+        # since MsSQL cannot set a session timezone.
+        # For timezone-naive columns (datetime/datetime2), no conversion is possible
+        # without knowing the source timezone — values are used as-is.
+        if isinstance(coltype, TimestampTZ):
+            value = f"{value} AT TIME ZONE 'UTC'"
+
         if coltype.precision > 0:
             formatted_value = (
                 f"FORMAT({value}, 'yyyy-MM-dd HH:mm:ss') + '.' + "