docs: explain endpoint classification with Structured Result

[JJasonSun]

Summary

• Document endpoint classification with a per-recipient Structured Result, including SDK and HTTP result paths.
• Add Calls guide coverage for the new example and null behavior.

API contract

Not changed.

Checklist

• I kept the change focused and public-facing.
• I removed secrets, private URLs, phone numbers, and customer data.
• Any OpenAPI change describes behavior supported by the public API.
• I updated tests when observable site behavior changed.
• No screenshot is required for this guide-only change.
• I ran pnpm run validate.