fix(skill): resolve Windows path compatibility issue in FileSystemSkillRepository

[lifl00001]

Description

Fixes Windows compatibility issue where FileSystemSkillRepository.getSource() returns source identifiers containing colons (e.g., filesystem:resources_skills), which causes InvalidPathException when used with
Path.resolve() on Windows file systems.

Fixes: Windows users can now use the Skill system without path errors.

Root Cause

On Windows, the colon (:) is a reserved character that can only be used for drive letters (e.g., C:). When Path.resolve() encounters a path string containing a colon, the Windows file system validator throws
InvalidPathException.

The error occurs in this call chain:
FileSystemSkillRepository.getSource() → returns "filesystem:resources_skills"
↓
AgentSkill.getSkillId() → returns "skill-name_filesystem:resources_skills"
↓
SkillBox.uploadSkillFiles() → calls targetDir.resolve(skillId)
↓
InvalidPathException: Illegal char <:> at index X

Changes

• FileSystemSkillRepository.java: Changed separator from : to _ in getSource() method
• FileSystemSkillRepositoryTest.java: Updated 5 test cases to expect new format

Format Change

Before	After
filesystem:parent_child	filesystem_parent_child

Platform Impact

Platform	Before	After
Windows	❌ InvalidPathException	✅ Works
Linux	✅ Works	✅ Works
macOS	✅ Works	✅ Works

Testing

• Updated unit tests pass
• mvn spotless:apply executed
• All existing tests continue to pass

Checklist

• Code follows project style guidelines (Spotless)
• Self-review completed
• Changes are limited to the scope of the fix
• Corresponding tests updated

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1 out of 2 committers have signed the CLA.

✅ lifl00001
❌ flli10

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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[lifl00001]

CLA signed

[lifl00001]

I have signed the CLA at https://cla-assistant.io/agentscope-ai/agentscope-java?pullRequest=915. Please recheck.