fix(providers/openai): zod parse error with function (vercel#6074) (vercel#6095)

This resolves an issue a user ran into:
vercel#6041

Aligned openai provider with openai-compatible

Author: samdenty

.changeset/unlucky-cherries-rescue.md

@@ -0,0 +1,5 @@
+---
+'@ai-sdk/openai': patch
+---
+
+fix(providers/openai): zod parse error with function

packages/openai/src/openai-chat-language-model.ts

@@ -713,7 +713,7 @@ const openaiChatChunkSchema = z.union([
                 z.object({
                   index: z.number(),
                   id: z.string().nullish(),
-                  type: z.literal('function').optional(),
+                  type: z.literal('function').nullish(),
                   function: z.object({
                     name: z.string().nullish(),
                     arguments: z.string().nullish(),