Support structureOutput from MCP server tools

[akhromow]

MCP server callTool function only returns the content field of a tool call response.
This creates a limitation: if an MCP tool returns only structuredOutput, an agent ends up receiving an empty array as the result.

const parsed = CallToolResultSchema.parse(response);
const result = parsed.content; // structuredOutput is ignored

This feature is being implemented already in python sdk, as use_structured_content parameter on MCPServer class.

[seratch]

Python implementation: openai/openai-agents-python#1150

[SkinnnyJay]

🚑 #471