fix(openai): allow open_page action type url to be nullish

[aayush-kapoor]

Background

OAI sometimes doesn't render the url param for the open_page action type, leading to validation errors.

Summary

Set the parameter to .nullish()

Manual Verification

• First, just run the repro code example to observe the error.
• Then apply the fix and run the same example again

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 #9308 and #10669