[Studio] fix: validate alert rule updates

[yx9o]

What is the purpose of the change

Fixes #574 .

Prevent /api/alert-rules/update from returning an internal server error for missing IDs or creating new rules for blank and unknown IDs.

This complements #549, which validates alert rule toggle and delete request bodies; this change covers update semantics.

Brief changelog

• Reject null and blank alert rule IDs with HTTP 400.
• Reject unknown alert rule IDs with HTTP 404.
• Atomically replace existing rules so update cannot create a missing rule.
• Add service and repository regression tests.