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

[noezhiya-dot]

The fix is a simple line swap but the issue it addresses is important. Two comments:

1. The core change (swapping lines 1125-1126) is correct — registering the base Parser override before the TSQL-specific one ensures the custom _parse_if wrapper is applied in the right order.

2. The test for PRINT strips the quotes from 'hello' — the expected output shows PRINT hello without quotes. Is this intentional? It seems like the T-SQL parser might be stripping string literal quotes from PRINT statements. If this is expected behavior, a comment in the test would help future readers understand why.

3. Consider adding a test that verifies the original error case from issue #5823 (the Required keyword: 'true' missing error) no longer occurs, to make the regression test more explicit.

[noezhiya-dot]

Clean fix — the root cause explanation makes sense, and swapping the override order is the minimal correct change. Tests cover the specific T-SQL case well. LGTM.

[jagannalla]

Thanks for the review, @noezhiya-dot!

I have updated the PR to address all comments and successfully force-pushed:

• Test Case: Added an explicit regression test to verify that the original error case from issue Required keyword: 'true' missing for <class 'sqlglot.expressions.functions.If'> #5823 (the Required keyword: 'true' missing error) no longer occurs. It now correctly parses @IF into an Anonymous expression rather than exp.If.
• Comments: Added an explanatory comment in the test suite regarding SQLGlot's fallback Command parser stripping string literal quotes from unparsed commands like PRINT.

Ready for review/merge!