fix: skillBox prompt should merge into system prompts

[fang-tech]

AgentScope-Java Version

[The version of AgentScope-Java you are working on, e.g. 1.0.9, check your pom.xml dependency version or run mvn dependency:tree | grep agentscope-parent:pom(only mac/linux)]

Description

fix & close #845

Checklist

Please check the following items before code is ready to be reviewed.

• Code has been formatted with mvn spotless:apply
• All tests are passing (mvn test)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (e.g. links, examples, etc.)
• Code is ready for review

[Copilot]

Pull request overview

Fixes issue #845 where using skillBox() alongside sysPrompt() could produce multiple SYSTEM messages, which some model providers reject (e.g., “System message must be at the beginning.”).

Changes:

• Update SkillHook to merge the skill prompt into an existing SYSTEM message when present, instead of always prepending a new SYSTEM message.
• Adjust existing tests and add a new regression test for issue #845 behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
agentscope-core/src/main/java/io/agentscope/core/skill/SkillHook.java	Merges skill prompt into an existing SYSTEM message (or prepends one if none exists).
agentscope-core/src/test/java/io/agentscope/core/skill/SkillHookTest.java	Updates the “inject at first” test and adds a new test to validate merging behavior for #845.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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