[v10.2.x] Alerting: Alert rule constraint violations return as 400s in provisioning API

[grafana-delivery-bot]

Backport 39599fa from #76396

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What is this feature?

Fixes the status code on constraint violation errors.

We use database constraints to enforce things like rule name uniqueness. This will make the API return the correct status code when a user tries to create/modify rule to where it breaches this.

Which issue(s) does this PR fix?:
n/a

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.