Improvements to dast site GQL resolvers

What does this MR do?

This makes the following changes:

• uses more direct project resolution in mutations
• catch errors appropriately in mutations
• test clean-up

This was work done as part of !40088 (merged) which introduces stricter requirements on resolvers, but it can be cleanly extracted.

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