feat: integrate structured output validation in server

Add mandatory structured output validation in the handleToolCall flow.
This ensures that any tool result with a defined output schema is
validated before being sent to the client, as required by the MCP protocol.

Author: davidleitw

server/server.go

@@ -1034,6 +1034,9 @@ func (s *MCPServer) handleToolCall(
 		}
 	}
 
+	if err := tool.Tool.ValidateStructuredOutput(result); err != nil {
+		return mcp.NewToolResultErrorFromErr("tool output schema validation failed", err), nil
+	}
 	return result, nil
 }