fix(format): preserve model column dialect

[fresioAS]

Description

Summary

• Fixes regression introduced in fix(format): keep MODEL/AUDIT/METRIC header dialect-agnostic (#5773) #5864
• Keep SQLMesh MODEL metadata dialect-agnostic during formatting.
• Render the MODEL columns schema using the model's dialect.
• Preserve dialect-specific column types such as DATETIME2(6) for T-SQL and Fabric.
• Keep generic metadata values such as descriptions and booleans unchanged.

Context

MODEL headers were recently changed to use SQLGlot's generic generator so that
T-SQL would not rewrite SQLMesh boolean properties such as FALSE into
(1 = 0).

However, the columns property contains dialect-specific data types. Rendering
it with the generic generator changed DATETIME2(6) into TIMESTAMP(6) for
T-SQL and Fabric models.

This change keeps the generic header behavior while rendering only the
columns schema with the model dialect.

Test Plan

• Added regression coverage for both tsql and fabric.
• Verified descriptions remain string literals.
• Verified formatting booleans remain FALSE.
• Full tests/core/test_dialect.py suite passes: 161 tests.
• Ruff and mypy pass.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO