fix: cast DuckDB IS_NULLABLE to string in metadata query (#8607)

DuckDB's information_schema.columns returns IS_NULLABLE as a boolean,
but the ColumnDef struct expects a string ('YES'/'NO'). This caused
deserialization to fail with "invalid type: boolean, expected a string"
when expanding WM_INTERNAL_DB markers (e.g. COUNT) for Ducklake tables.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/windmill-common/src/query_builders.rs

@@ -2369,7 +2369,7 @@ fn make_load_table_metadata_query(
     COLUMN_DEFAULT as DefaultValue,
     false as IsPrimaryKey,
     false as IsIdentity,
-    IS_NULLABLE as IsNullable,
+    CASE WHEN IS_NULLABLE = true THEN 'YES' ELSE 'NO' END as IsNullable,
     false as IsEnum,
     TABLE_NAME as table_name
 FROM information_schema.columns c