fix(agent): hoist structured output defs to tool root

[Alexxigang]

Summary

• hoist nested $defs and legacy definitions to the generated generate_response tool schema root
• reuse the same schema normalization path for structured output wrapping and regular tool schema generation
• add regression coverage for nested dynamic JSON schemas, validation behavior, and input-schema immutability

Root cause

StructuredOutputCapableAgent wraps the response schema under a top-level response property when it registers the temporary generate_response tool. When the wrapped schema contains nested $ref values like #/$defs/Material, those references resolve from the tool schema document root, but the definitions were still nested under properties.response. That makes validation fail for multi-level structured output schemas.

Validation

• mvn -pl agentscope-core '-Dtest=JsonSchemaUtilsTest,StructuredOutputDynamicDefineTest,ToolValidatorTest' test -q
• mvn -pl agentscope-core '-Dtest=JsonSchemaUtilsTest,StructuredOutputDynamicDefineTest,ToolValidatorTest,ReActAgentStructuredOutputTest,StructuredOutputHookTest' test -q
• mvn -pl agentscope-core spotless:check -q