[FLINK-AGENTS][api] Fix Kryo deserialization failure for Event classes with immutable lists

[avichaym]

fixes #673

Wraps list arguments in new ArrayList<>() before passing to setAttr() in ChatRequestEvent, ContextRetrievalResponseEvent, and ToolRequestEvent. Without this, passing List.of(...) crashes Kryo on JDK 17+ with InaccessibleObjectException.

[wenjin272]

Hi, @avichaym, Ty for this fix.

The current fix looks good to me, but I think two additional points need to be added:

1. If user use Map.of to consturct ToolResponseEvent, there will likely be issues as well.
2. Supplement with relevant tests. Since in the CI pipeline, the test cases in flink-agents-end-to-end-tests-integration module will be executed in jdk 17 with flink 1.20, 2.0, 2.1 and 2.2. I think we can add a EventKryoSerializationTest in this module like:

@Test
void chatRequestEventWithImmutableListSurvivesKryoRoundTrip() throws Exception {
    ChatRequestEvent event =
            new ChatRequestEvent(
                    "myModel", List.of(new ChatMessage(MessageRole.USER, "hello")));

    ChatRequestEvent roundTripped = roundTrip(event, ChatRequestEvent.class);

    assertEquals("myModel", roundTripped.getModel());
    assertNotNull(roundTripped.getMessages());
    assertEquals(1, roundTripped.getMessages().size());
    assertEquals("hello", roundTripped.getMessages().get(0).getContent());
}
private static <T extends Event> T roundTrip(T event, Class<T> cls) throws Exception {
    KryoSerializer<T> serializer = new KryoSerializer<>(cls, new SerializerConfigImpl());
    DataOutputSerializer out = new DataOutputSerializer(64);
    serializer.serialize(event, out);
    DataInputDeserializer in = new DataInputDeserializer(out.getCopyOfBuffer());
    return serializer.deserialize(in);
}

[avichaym]

Thanks for the careful review @wenjin272 - both points addressed in the new commit :

1. ToolResponseEvent now defensive-copies all four maps (responses, success, error, externalIds) so Map.of(...) callers are safe - same shape as the list fix.

2. Added EventKryoSerializationTest under flink-agents-end-to-end-tests-integration covering all four affected events.

[wenjin272]

Currently, we only copy the outer ImmutableList/ImmutableMap without handling nested ImmutableList/ImmutableMap cases. Therefore, this test will fail under Flink 1.20.

I think there are probably two solutions to this problem:

1. Supports copying arbitrarily nested ImmutableList/ImmutableMap structures into nested MutableList/MutableMap, but this might be too heavy for constructing Events.
2. Since both ToolRequestEvent and ToolResponseEvent are constructed by built-in actions, users typically do not create these events themselves. Therefore, we forgo defensive checks for Tool Events.

I personally prefer option 2, WDYT？

[avichaym]

Agreed, option 2 makes more sense. Updated to revert the Tool event changes and keeping defensive copies only on ChatRequestEvent and ContextRetrievalResponseEvent. Test trimmed to those two cases.

[wenjin272]

LGTM