fix(event): avoid Jackson 3 type property conflict

[guslegend0510]

AgentScope-Java Version

2.0.0-SNAPSHOT

Description

Closes #1948.

This PR fixes io.agentscope.core.event.AgentEvent serialization failures under Jackson 3, especially when events are streamed through Spring MVC SseEmitter / ResponseBodyEmitter.

Root cause

AgentEvent used:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")

At the same time, the event hierarchy exposes getType(). Under Jackson 3 this causes a serializer conflict between the synthetic type-id property and the bean property named type, which leads to:

InvalidDefinitionException: Conflict between type id property 'type' and bean property with same name

Changes

• Change AgentEvent polymorphic type handling from JsonTypeInfo.As.PROPERTY to JsonTypeInfo.As.EXISTING_PROPERTY
• Keep the existing getType() contract as the single source of the serialized type field
• Add regression tests for:
  • AgentStartEvent serialization containing exactly one type field
  • polymorphic deserialization from AgentEvent base type

How to test

Run targeted regression tests:

mvn -pl agentscope-core test -Dtest=AgentEventStreamTest

I also ran:

mvn -pl agentscope-core test

The event/Jackson-related tests passed. Two existing Windows-specific symlink permission tests failed in my environment and appear unrelated to this change:

• DangerousPathBypassTest.symlinkToDotEnvIsDetected
• DangerousPathBypassTest.symlinkToSshIsDetected

Checklist

• Code has been formatted with mvn spotless:apply
• All relevant tests are passing (mvn -pl agentscope-core test -Dtest=AgentEventStreamTest)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (not needed for this internal serialization fix)
• Code is ready for review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[itxaiohanglover]

Clean fix. Switching from As.PROPERTY to As.EXISTING_PROPERTY resolves the Jackson conflict between the type-id property and the bean property named type. The test coverage for both serialization (single type property) and deserialization (polymorphic) looks solid.