修复 AutoContext 中 system 消息混入 inputMessages 的问题

[guslegend0510]

背景

当前 AutoContext 在构造事件时，会把 system 说明和普通输入一起放进 inputMessages。这样容易出现多个 system turn 混杂、上下文语义不清或推理阶段读取冲突的问题。

修改内容

• 将 system 说明从 inputMessages 中剥离出来。
• 统一写入 PreReasoningEvent.systemMessage。
• 让 inputMessages 只保留非 system turn，降低后续处理歧义。
• 避免多 system 消息带来的上下文冲突。

验证

• mvn -pl agentscope-extensions/agentscope-extensions-autocontext-memory -am -Dtest=AutoContextHookTest test

Fixes #1484

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[codecov]

Codecov Report

❌ Patch coverage is 73.68421% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...scope/core/memory/autocontext/AutoContextHook.java	73.68%	1 Missing and 4 partials ⚠️

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