> AWS should provide features where you are prompted to set a hard budget limit when you first signup and services fail once that's exceeded

They should, but the entire internal metering & bill generation pipeline isn't setup to support that. You can configure billing alerts but these will only refresh on updates to your estimated bill (typically re-computed every 8hrs). And since service teams are responsible for posting metering records to commerce platform their implementation may vary.

TLDR: AWS Commerce Platform is a mess.

Source: Ex-AWS SDE