fix(skill): ignore complex YAML frontmatter to prevent parsing errors

[jujn]

Description

Close #1030

This PR updates the internal SimpleYamlParser to gracefully skip unsupported block-style complex structures (like lists or nested objects) instead of throwing exceptions.

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

This PR makes MarkdownSkillParser more tolerant of complex/invalid YAML frontmatter by skipping unsupported lines instead of throwing, so skills with non-flat frontmatter won’t block loading.

Changes:

• Updated SimpleYamlParser to skip non-key: value YAML lines and avoid throwing on unsupported structures.
• Added/updated unit tests to assert “graceful ignore” behavior for invalid lines and list-style YAML in frontmatter.
• Adjusted empty-value generation/parsing expectations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
agentscope-core/src/main/java/io/agentscope/core/skill/util/MarkdownSkillParser.java	Makes YAML parsing tolerant by skipping unsupported lines/structures and adds debug logging.
agentscope-core/src/test/java/io/agentscope/core/skill/util/MarkdownSkillParserTest.java	Updates tests to validate non-throwing behavior and adds a complex-frontmatter parsing scenario.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...gentscope/core/skill/util/MarkdownSkillParser.java	86.66%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[jujn]

@fang-tech PTAL Thanks.