feat(harness): Add the middleware function for dialogue suggestions

[Zzzz-zmy]

AgentScope-Java Version

[The version of AgentScope-Java you are working on, e.g. 1.0.12, check your pom.xml dependency version or run mvn dependency:tree | grep agentscope-parent:pom(only mac/linux)]

Description

• Register the SUGGESTION_RESULT event type in AgentEvent
• Add the SuggestionResultEvent event class to carry suggestion results
• Implement the SuggestionMiddleware middleware to generate follow-up question suggestions
• Add configuration options enableSuggestions and suggestionMaxItems
• Integrate the suggestion middleware into HarnessAgent
• Add comprehensive unit tests to verify the suggestion functionality

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

[CLAassistant]

All committers have signed the CLA.

[Zzzz-zmy]

close #1975

[codecov]

Codecov Report

❌ Patch coverage is 63.42857% with 64 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...harness/agent/middleware/SuggestionMiddleware.java	63.15%	31 Missing and 25 partials ⚠️
...java/io/agentscope/harness/agent/HarnessAgent.java	25.00%	5 Missing and 1 partial ⚠️
...o/agentscope/core/event/SuggestionResultEvent.java	85.71%	1 Missing and 1 partial ⚠️

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