Fix Kotlin nullable fields in MCP tool input schema

[qixiangme]

Summary

This PR is related to #5978 and fixes the MCP Tools schema generation path for Kotlin nullable fields.

PR #5897 addresses the standard @Tool schema path by registering org.springframework.ai.model.KotlinModule in JsonSchemaGenerator. MCP tools use McpJsonSchemaGenerator for method input schemas, so Kotlin nullable properties inside MCP tool parameter data classes could still be emitted as required in nested JSON Schema objects.

This change applies the same conditional KotlinModule registration pattern to McpJsonSchemaGenerator, keeping the fix narrowly scoped to MCP tool input schema generation.

Tests

The regression test verifies both sides of Kotlin requiredness behavior:

• nullable Kotlin constructor properties with default null values, such as String? = null and List<String>? = null, are not included in nested required arrays
• non-null Kotlin constructor properties without defaults remain required

Commands run:

./mvnw -pl mcp/mcp-annotations '-Denforcer.skip=true' 'kotlin:test-compile@test-compile'
./mvnw -pl mcp/mcp-annotations -Dtest=McpJsonSchemaGeneratorKotlinTests '-Denforcer.skip=true' surefire:test
./mvnw -pl mcp/mcp-annotations -Dtest=McpJsonSchemaGeneratorKotlinTests '-Denforcer.skip=true' package

Related issue: Addresses the MCP Tools portion of #5978.

[sdeleuze]

FYI The error is caused by a javac behavior described in openjdk/jdk#28018, I will refine to fix it.

[qixiangme]

Thanks for the clarification. I'll keep an eye on the updated fix.