Pass dialect string (not Dialect instance) to symmetric aggregate builder

build_symmetric_aggregate_sql branches on dialect name strings like
"bigquery", "postgres", etc. Passing self._dialect_instance (a Dialect
object) would miss all branches and fall back to DuckDB SQL, producing
invalid SQL for non-DuckDB engines.

Author: nicosuave

sidemantic/sql/generator.py

@@ -1912,7 +1912,7 @@ def _build_main_select(
                                 primary_key=pk,
                                 agg_type=measure.agg,
                                 model_alias=f"{model_name}_cte",
-                                dialect=self._dialect_instance,
+                                dialect=self.dialect,
                             )
                         else:
                             # Use helper that applies metric-level filters via CASE WHEN