bug(phai): PostHog AI tries to use non-supported hogql functions like toDateTime64

### Bug Description

PHAI generates and tries to use invalid HogQL, resulting in an error

## How to reproduce

See screenshot in convo

## Additional context

https://posthog.slack.com/archives/C0AV33BDCJ3/p1777928656159119

### Debug info

```shell

```