Support binary API responses

[Tsopic]

Summary

• Detect binary success responses from the loaded OpenAPI operation and request them as Axios arraybuffer data.
• Return image/* downloads as MCP image content.
• Return PDF and other binary downloads as embedded MCP resources with MIME type and base64 data.
• Preserve JSON/text success responses and structured JSON/text errors, including on binary-capable operations.

This makes the REST Files API download operation usable through MCP instead of JSON-stringifying or corrupting binary response bytes.

Closes #15.

Testing

• npm run lint
• npm run typecheck
• npm test — 9 files, 95 tests passed
• npm run build

New coverage verifies direct and referenced binary OpenAPI responses, Axios binary response configuration, image/PDF content, MIME handling, base64 encoding, JSON success variants, and structured JSON/text errors.