refactor: move extraHTTPHeaders into emulate tool

Address review feedback to extend the existing emulate tool
instead of adding a standalone set_extra_http_headers tool.

- Add extraHTTPHeaders param to emulate tool schema
- Add handler logic in McpContext.emulate()
- Preserve existing headers when param is omitted
- Move tests from network.test.ts to emulation.test.ts
- Revert network.ts and network.test.ts to main

Author: pedrodurek