Modify invalid Snippet response status codes

What does this MR do?

The Snippets::BaseService currently handles invalid parameters by returning a ServiceResponse with a status code of 403 (forbidden).

This status doesn't feel applicable in these scenarios, so this MR is to adjust the status to a 422 (Unprocessable Entity) and add some test coverage.

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