Is your feature request related to a problem? Please describe.

PR #71 made Express optional, but the SDK still can't run on edge runtimes
(Cloudflare Workers, Deno, Bun) because:

1. DefaultExecutionEventBus uses Node.js EventEmitter
2. Extension utilities (HTTP_EXTENSION_HEADER, getRequestedExtensions) aren't exported

This forces anyone wanting Hono/Fastify support to maintain a full fork.

Describe the solution you'd like

1. Replace EventEmitter with EventTarget

• Web-standard API available in all modern runtimes
• Node.js 15+ has native EventTarget support
• Maintains same public API

2. Export extension utilities

// In src/server/index.ts
export { HTTP_EXTENSION_HEADER } from '../constants.js';
export { getRequestedExtensions } from './utils.js';

Benefits

• ✅ Native Cloudflare Workers support (no nodejs_compat)
• ✅ Native Deno/Bun support
• ✅ Browser compatibility
• ✅ Enables community framework adapters
• ✅ Smaller bundle for edge deployments

Describe alternatives you've considered

I've implemented this in my fork: https://github.com/drew-foxall/a2a-js-sdk

Additional context

I've also used my fork to create working implementations using hono to create alternate versions of the a2a-examples https://github.com/drew-foxall/a2a-js-sdk-examples

Code of Conduct

• I agree to follow this project's Code of Conduct