Validate Prometheus alert rules at model level

What does this MR do?

The prometheus_alerts table has not-null constraints for operator and threshold fields, preventing invalid entries from being saved. This raises a ActiveRecord::NotNullViolation, but we're save-ing the model rather than save!-ing, so it's better to add a validation rather than wait for an error.

This is not a user-facing change, so no changelog entry.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team