Support generic output schemas in async MCP tool providers

[wxggzz]

Summary

Generate MCP output schemas for parameterized reactive return types in both stateful and stateless async tool providers.

When an @McpTool(generateOutputSchema = true) method returns a type such as Mono<List<Result>>, the reactive payload is a ParameterizedType. Both async providers converted non-Class<?> payloads to null before calling ClassUtils, causing tool registration to fail with IllegalArgumentException: Class must not be null.

This change:

• preserves the existing primitive and simple-value filtering for class payloads;
• generates schemas from ParameterizedType payloads;
• covers both stateful and stateless async providers;
• verifies the generated array schema and the structured tool result.

This supersedes and extends #5920 by covering both async provider variants and adding end-to-end structured-content assertions.

Testing

• ./mvnw -Dmaven.build.cache.enabled=false -pl mcp/mcp-annotations -am test (1,378 tests, 0 failures, 2 skipped)
• ./mvnw clean package (169 reactor modules, BUILD SUCCESS)