feat: restore voice-call plugin parity

extensions/voice-call/src/providers/index.ts

@@ -0,0 +1,9 @@
+export type { VoiceCallProvider } from "./base.js";
+export { MockProvider } from "./mock.js";
+export {
+  OpenAIRealtimeSTTProvider,
+  type RealtimeSTTConfig,
+  type RealtimeSTTSession,
+} from "./stt-openai-realtime.js";
+export { TelnyxProvider } from "./telnyx.js";
+export { TwilioProvider } from "./twilio.js";