ref(server-utils): Remove array JSON-stringify fallback in Vercel AI integration (#23102)

We now support arrays so we don't need this fallback stringification for
any arrays in vercel. Conventions still require the attributes that we
send to be strings so keeping the explicit attribute stringifications
and adding one for the `finish_reason` so we preserve the format.

Fixes #20863

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: nicohrubec

packages/server-utils/src/ai/vercel-ai/index.ts

@@ -17,6 +17,7 @@ import {
   GEN_AI_OUTPUT_MESSAGES,
   GEN_AI_PROVIDER_NAME,
   GEN_AI_REQUEST_MODEL,
+  GEN_AI_RESPONSE_FINISH_REASONS,
   GEN_AI_RESPONSE_MODEL,
   GEN_AI_TOOL_CALL_ARGUMENTS,
   GEN_AI_TOOL_CALL_RESULT,
@@ -334,14 +335,13 @@ export function processVercelAiSpanAttributes(attributes: Record<string, unknown
     attributes[GEN_AI_EMBEDDINGS_INPUT] = parsed.length === 1 ? parsed[0] : JSON.stringify(parsed);
   }
 
+  if (Array.isArray(attributes[GEN_AI_RESPONSE_FINISH_REASONS])) {
+    attributes[GEN_AI_RESPONSE_FINISH_REASONS] = JSON.stringify(attributes[GEN_AI_RESPONSE_FINISH_REASONS]);
+  }
+
   addProviderMetadataToAttributes(attributes);
 
   for (const key of Object.keys(attributes)) {
-    // JSON-stringify any array-valued attributes so they survive v2 span serialization.
-    // Can be removed once span streaming supports arrays natively.
-    if (Array.isArray(attributes[key])) {
-      attributes[key] = JSON.stringify(attributes[key]);
-    }
     // Change attributes namespaced with `ai.X` to `vercel.ai.X`
     if (key.startsWith('ai.')) {
       renameAttributeKey(attributes, key, `vercel.${key}`);