Extend callTool() to support latest MCP capabilities.

[GnikDroy]

As of 2025-06-18, MCP server tool responses have additional fields that are not returned by callTool().

Currently callTool() returns CallToolResponse which has the following structure.

export interface CallToolResponse extends JsonRpcResponse {
    result: {
        content: {
            type: string;
            text: string;
        }[];
    };
}

The specification has evolved to support structuredContent, explicit error handling with isError and an optional outputSchema.

callTool() should return all this additional information to the caller.