Partial aggregation over long decimals fails if pushed below join

[findepi]

Query failed: class io.trino.spi.block.DictionaryBlock cannot be cast to class io.trino.spi.block.VariableWidthBlock

Caused by: java.lang.ClassCastException: class io.trino.spi.block.DictionaryBlock cannot be cast to class io.trino.spi.block.VariableWidthBlock (io.trino.spi.block.DictionaryBlock and io.trino.spi.block.VariableWidthBlock are in unnamed module of loader 'app')
 	at io.trino.operator.aggregation.state.LongDecimalWithOverflowStateSerializer.deserialize(LongDecimalWithOverflowStateSerializer.java:67)
 	at io.trino.operator.aggregation.state.LongDecimalWithOverflowStateSerializer.deserialize(LongDecimalWithOverflowStateSerializer.java:26)
 	at io.trino.$gen.sumGroupedAccumulator_20240313_131249_42.addIntermediate(Unknown Source)
 	at io.trino.operator.aggregation.GroupedAggregator.processPage(GroupedAggregator.java:93)
 	at io.trino.operator.aggregation.builder.InMemoryHashAggregationBuilder.lambda$processPage$1(InMemoryHashAggregationBuilder.java:157)
 	at io.trino.operator.TransformWork.process(TransformWork.java:44)
 	at io.trino.operator.HashAggregationOperator.addInput(HashAggregationOperator.java:443)
 	at io.trino.operator.Driver.processInternal(Driver.java:400)

[findepi]

this is apparently a regression in 440 (#19816)

can be reproduced

Session session = Session.builder(assertions.getDefaultSession())
        .setSystemProperty(PUSH_PARTIAL_AGGREGATION_THROUGH_JOIN, "true")
        .build();
assertThat(assertions.query(session, """
        SELECT l.i, sum(v)
        FROM (VALUES 1, 2, 2, 3, 3, 3) l(i)
        JOIN (VALUES
            (1, DECIMAL '12345678901234567890.1234567890'),
            (1, DECIMAL '11111111111111111111.1234567890'),
            (2, DECIMAL '22222222222222222222.1234567890'),
            (3, DECIMAL '33333333333333333333.1234567890'),
            (7, DECIMAL '77777777777777777777.1234567890')) r(i, v) ON l.i = r.i
        GROUP BY l.i
        """))
        .matches("""
                VALUES
                    (1, CAST(DECIMAL '23456790012345679001.2469135780' AS decimal(38, 10))),
                    (2, CAST(DECIMAL '44444444444444444444.2469135780' AS decimal(38, 10))),
                    (3, CAST(DECIMAL '99999999999999999999.3703703670' AS decimal(38, 10)))
                """);