fix(agent): hoist structured output defs to root

[Alexxigang]

Summary

• hoist nested $defs / definitions to the root when wrapping structured-output schemas under the temporary generate_response tool
• keep the wrapped
  esponse schema focused on its object body so #//... references resolve correctly during tool parameter validation
• add regression tests for reused nested objects both at the schema-validation layer and through an actual ReActAgent structured-output flow

Why this fix

Issue #1176 happens because StructuredOutputCapableAgent wraps the generated output schema under a
esponse property, but leaves shared definitions nested inside that property. The generated $ref values still point to #//..., which means validation fails once the schema is wrapped and the definitions are no longer located at the document root.

This change mirrors the same def-hoisting strategy already used in tool parameter schema generation, so reusable nested objects continue to work after the structured-output schema is wrapped for generate_response.

Validation

• mvn -pl agentscope-core spotless:apply
• mvn -pl agentscope-core '-Dtest=StructuredOutputSchemaWrappingTest' test
• mvn -pl agentscope-core '-Dtest=ReActAgentStructuredOutputTest,ToolkitTest,ToolValidatorTest,JsonSchemaUtilsTest' test

Notes

• This also appears to address the same unresolved-reference symptom reported in [Bug]:结构化输出，会调用generate_response工具，但是如果存在多层嵌套，工具会报错 #1175.

AI Assistance

• Drafted with AI assistance and validated locally before submission.