Use standard Headers API instead of custom request context object #1300

Summary

Replace custom request context objects with the standard web Headers API for better standards alignment and developer familiarity.

This was identified as a gap in the v2 wishlist (issue #809) that is not yet tracked as a separate issue.

Motivation

• Standards alignment: The Headers API is a web standard that developers already know
• Interoperability: Standard APIs work better across different runtimes (Node.js, Deno, Bun, browsers)
• Developer experience: Familiar API reduces learning curve
• Type safety: Headers has well-defined TypeScript types

Requirements

1. Replace custom header/context objects with standard Headers API
2. Update request context interfaces to use Headers
3. Ensure compatibility with Node.js, Deno, Bun runtimes
4. Provide migration guidance for existing code

API Changes

// Before (custom object)
interface RequestContext {
  headers: Record<string, string>;
  // ...
}

// After (standard Headers)
interface RequestContext {
  headers: Headers;
  // ...
}

Related Issues

• SDK V2 #809 (SDK V2 wishlist)