[Google AI Overview API] No AI Overview generated sporadically

[NateSkiles]

A couple customers reached out on #1622 reporting queries that had previously returned AI Overviews are now returning an error:

"ai_overview": {
    "error": "Can't generate an AI overview right now. Try again later."
},

This query returns an AI Overview in Google:

Google Search

This only appears to be affecting specific queries, as q=drop+shipping still works (playground):

Playground 1 | Playground 2 | Documentation

Search Inspect 1 | Search Inspect 2