feat(ai): expose routed model on openai-completions (#3968)

Adds `AssistantMessage.responseModel` on the openai-completions path:
surfaces the concrete `chunk.model` when it differs from the requested
id (e.g. OpenRouter `auto` -> `anthropic/...`).

packages/ai/src/providers/openai-completions.ts

@@ -207,6 +207,9 @@ export const streamOpenAICompletions: StreamFunction<"openai-completions", OpenA
 				// OpenAI documents ChatCompletionChunk.id as the unique chat completion identifier,
 				// and each chunk in a streamed completion carries the same id.
 				output.responseId ||= chunk.id;
+				if (typeof chunk.model === "string" && chunk.model.length > 0 && chunk.model !== model.id) {
+					output.responseModel ||= chunk.model;
+				}
 				if (chunk.usage) {
 					output.usage = parseChunkUsage(chunk.usage, model);
 				}

packages/ai/src/types.ts

@@ -216,6 +216,7 @@ export interface AssistantMessage {
 	api: Api;
 	provider: Provider;
 	model: string;
+	responseModel?: string; // Concrete `chunk.model` when different from the requested `model` (e.g. OpenRouter `auto` -> `anthropic/...`)
 	responseId?: string; // Provider-specific response/message identifier when the upstream API exposes one
 	usage: Usage;
 	stopReason: StopReason;