docs: clarify Calls API failure outcome semantics

[JJasonSun]

Summary

• distinguish Developer API request errors, accepted Calls outcomes, and Goal Run errors
• document that Calls API failure_code is a nullable diagnostic string without a published enum
• clarify that integrations must not infer no answer or callee decline from undocumented Calls API strings
• link the Calls guide to the canonical accepted-outcome guidance
• add browser regression coverage for the link, anchor, and unresolved-outcome warning

API contract

Not changed.

Related issues

• Relates to Document Calls API failure codes and no-answer mapping awesome-phone-call-agents#89
• Relates to Return precise call failure codes and distinguish platform failures from callee declines awesome-phone-call-agents#206

Validation

• pnpm run validate
  • TypeScript typecheck passed
  • production build and static distribution verification passed
  • 11 deployment unit tests passed
  • 18 Playwright browser tests passed

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.
• Screenshots are not applicable because this change has no visual changes.
• I ran pnpm run validate.