Fix NPE and add support for generic return types in AsyncMcpToolProvider

[luoshengming]

This PR fixes a bug in AsyncMcpToolProvider where using generic return types (e.g., Mono<ApiResponse<String>>) would cause an IllegalArgumentException due to passing null to ClassUtils.isPrimitiveOrWrapper().

Problem

When a tool method returns a parameterized type like Mono<ApiResponse<T>>, the typeArgument extracted from the return type is a ParameterizedType, not a Class<?>. The original code would set methodReturnType to null and then pass it to ClassUtils.isPrimitiveOrWrapper(), which throws IllegalArgumentException: Class must not be null.

Solution

• Handle Class<?> and ParameterizedType separately using pattern matching
• For ParameterizedType, use McpJsonSchemaGenerator.generateFromType() instead of generateFromClass()
• This aligns with how SyncMcpToolProvider handles generic return types

Testing

Added test case testGetToolSpecificationsWithGenericReturnType() that verifies generic return types are properly handled and output schema is correctly generated.

Impact

• ✅ Fixes a real bug affecting users with generic return types
• ✅ Backward compatible - no breaking changes
• ✅ Aligns async implementation with sync implementation