Fixes#4038
The global fetch in Node.js doesn't support undici's dispatcher option,
which is required for ProxyAgent to work. This fix imports fetch from
undici directly to enable proper proxy support for Telegram API calls.
Root cause: makeProxyFetch() was using global fetch with { dispatcher: agent },
but Node.js's global fetch ignores the dispatcher option. Using undici.fetch
ensures the ProxyAgent dispatcher is properly respected.
Tested: Build passes, TypeScript compilation successful.
