Fix(ai): correctly valiate empty arrays

[AVtheking]

Background

The validateUIMessages and safeValidateUIMessages functions don't properly check for arrays being not-empty,

Summary

Updated the zod schema to catch empty arrays passed in messages.

Manual Verification

Wrote tests with empty arrays throwing errors, this should be sufficient right ?

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Related Issues

Fixes #10003

[gr2m]

I wonder if this is worth validating with zod, or if we should update this check here (or add a new check)

ai/packages/ai/src/ui/validate-ui-messages.ts

Line 324 in 81d4308

if (messages == null) {

[AVtheking]

I wonder if this is worth validating with zod, or if we should update this check here (or add a new check)

ai/packages/ai/src/ui/validate-ui-messages.ts

Line 324 in 81d4308

if (messages == null) {

Yeah could update here also, but I thought zod is already being used for validation so it should be good.

Also this null check would also be caught by zod isn't it ? I assume reason being checking this separately is to provide clear error message, which is for non empty array is provided by zod

[gr2m]

you know what let's just go ahead. If for some reason a dedicated check will be better we can change that later

[vercel-ai-sdk]

⚠️ Backport to release-v5.0 created but has conflicts: #10117