Allow async streamFn for dynamic proxy settings

- StreamFn type now allows returning Promise
- agent-loop awaits streamFn result
- createStreamFn takes getProxyUrl callback, reads settings on each call

packages/agent/src/agent-loop.ts

@@ -211,7 +211,7 @@ async function streamAssistantResponse(
 	const resolvedApiKey =
 		(config.getApiKey ? await config.getApiKey(config.model.provider) : undefined) || config.apiKey;
 
-	const response = streamFunction(config.model, llmContext, {
+	const response = await streamFunction(config.model, llmContext, {
 		...config,
 		apiKey: resolvedApiKey,
 		signal,