fix: Align SqlTable::supports_filters_pushdown with scan_to_sql by using Unparser

[sgrebnov]

SqlTable::supports_filters_pushdown() used expr::to_sql_with_engine() to check filter pushdown capability, while scan_to_sql() used Unparser::expr_to_sql() for actual SQL generation. The custom converter has a narrow match with a _ => Err catch-all that rejects valid expressions like CASE WHEN, IS NULL, and IS NOT NULL — even though the Unparser handles them correctly.

This mismatch causes filters to be incorrectly reported as Unsupported, preventing pushdown of expressions that scan_to_sql() can convert to SQL.

Changes:

• Switch supports_filters_pushdown() to use Unparser::expr_to_sql() with the table's dialect, matching the converter used by scan_to_sql()
• Reject subqueries (Exists, InSubquery, ScalarSubquery) and OuterReferenceColumn before the Unparser check, since the Unparser can convert these to SQL text but they must not be pushed into TableScan.full_filters — subqueries reference tables potentially in other databases, and outer references belong to an enclosing query the table provider cannot resolve