fix: force telegram native fetch under bun

src/telegram/fetch.ts

@@ -0,0 +1,8 @@
+// BAN compatibility: force native fetch to avoid grammY's node-fetch shim under Bun.
+export function resolveTelegramFetch(proxyFetch?: typeof fetch): typeof fetch {
+  const fetchImpl = proxyFetch ?? globalThis.fetch;
+  if (!fetchImpl) {
+    throw new Error("fetch is not available; set telegram.proxy in config");
+  }
+  return fetchImpl;
+}