workload/schemachange: fix date parsing error in SET DEFAULT operations

The schemachange workload could generate invalid SQL when testing type
mismatches, causing parse errors instead of expected type mismatch errors.

Fix by always using explicit type casting in SET DEFAULT statements.

Fixes: #152572

Release note: None

Epic: None

Author: spilchen

pkg/workload/schemachange/operation_generator.go

@@ -2530,8 +2530,11 @@ func (og *operationGenerator) setColumnDefault(ctx context.Context, tx pgx.Tx) (
 	}
 
 	strDefault := tree.AsStringWithFlags(defaultDatum, tree.FmtParsable)
-	stmt.sql = fmt.Sprintf(`ALTER TABLE %s ALTER COLUMN %s SET DEFAULT %s`,
-		tableName.String(), columnForDefault.name.String(), strDefault)
+	// Always use explicit type casting to ensure consistent behavior and avoid parse errors.
+	// When the types don't match, this will produce the expected DatatypeMismatch error.
+	// When the types do match, the cast is harmless and makes the intent explicit.
+	stmt.sql = fmt.Sprintf(`ALTER TABLE %s ALTER COLUMN %s SET DEFAULT %s::%s`,
+		tableName.String(), columnForDefault.name.String(), strDefault, datumTyp.SQLString())
 	return stmt, nil
 }