Add tests for error case where a required field is missing when visiting

[sebastiantia]

Please describe why this is necessary.

Currently, none of the tests in kernel/src/actions/visitor.rs verify error cases where a required field (e.g., add.path) is missing. This means we are not asserting expected failures in cases where input data is malformed or incomplete.

Describe the functionality you are proposing.

Define a custom error type VisitorError to represent failure cases more explicitly.

Add tests asserting for failures in cases where action data is malformed or incomplete.

Additional context

No response