fix(ai): support OpenRouter session affinity (#6496)

Fixes #6366

packages/ai/test/openai-completions-thinking-as-text.test.ts

@@ -39,6 +39,7 @@ const compat = {
 	supportsStrictMode: true,
 	cacheControlFormat: undefined,
 	sendSessionAffinityHeaders: false,
+	sessionAffinityFormat: "openai",
 	supportsLongCacheRetention: true,
 } satisfies Required<Omit<OpenAICompletionsCompat, "cacheControlFormat">> & {
 	cacheControlFormat?: OpenAICompletionsCompat["cacheControlFormat"];