标题：修复 AutoContext 压缩助手消息被误判为最终回复

[guslegend0510]

背景

AutoContext 压缩后会生成带 _compress_meta 的 assistant 消息，这类消息属于合成消息，不应作为对话的最终自然回复。原逻辑会把它们算作 final assistant response，影响后续上下文判断。

修改内容

• 将任意携带 _compress_meta 的 assistant 消息统一视为 synthetic message。
• 在 final assistant response 判定中排除此类消息。
• 让最终回复识别只依赖真实对话输出，避免压缩过程干扰结果。

验证

• mvn -pl agentscope-extensions/agentscope-extensions-autocontext-memory -am "-Dtest=MsgUtilsTest,AutoContextMemoryTest" test

Fixes #1483

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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