feat: custom provider support with streamSimple

- Add resetApiProviders() to clear and re-register built-in providers
- Add createAssistantMessageEventStream() factory for extensions
- Add streamSimple support in ProviderConfig for custom API implementations
- Call resetApiProviders() on /reload to clean up extension providers
- Add custom-provider.md documentation
- Add custom-provider.ts example with full Anthropic implementation
- Update extensions.md with streamSimple config option

packages/ai/src/utils/event-stream.ts

@@ -80,3 +80,8 @@ export class AssistantMessageEventStream extends EventStream<AssistantMessageEve
 		);
 	}
 }
+
+/** Factory function for AssistantMessageEventStream (for use in extensions) */
+export function createAssistantMessageEventStream(): AssistantMessageEventStream {
+	return new AssistantMessageEventStream();
+}