fix: Swap parser override order for macro IF under T-SQL (Issue #5823)

[jagannalla]

Summary

This PR fixes a bug where conditional macro statements (e.g. @IF(cond, statement)) using the T-SQL / MSSQL dialect would fail validation with:
Required keyword: 'true' missing for <class 'sqlglot.expressions.functions.If'>

Root Cause

In extend_sqlglot() within sqlmesh/core/dialect.py, the overrides were registered in the wrong order:

_override(TSQL.Parser, Parser._parse_if)
_override(Parser, _parse_if)

Solution

1. Swapped the registration lines in sqlmesh/core/dialect.py so the custom parser wrapper is registered on the base Parser first.
2. Added unit tests under tests/core/test_dialect.py for T-SQL-specific statements parsed within @IF.

Link to Issue

Closes #5823