fix: check if catalog type is editable before deletion

[maxtacu]

small context

We let people manage schedules directly in incident.io, but want governance over naming and enforce conventions. We achieve this by initializing empty schedules via Terraform using the incident_schedule resource, with lifecycle rules to ignore changes except the name. This lets people init schedules through Git and approval, then edit them in incident.io. We also set a custom boolean catalog entry attribute to mark if a schedule is paid, managed in the incident_catalog_entry resource.

problem statement:

When destroying a schedule in Terraform, the provider tries to delete the linked catalog entry first. But for externally synced catalog types (like the built-in "Schedule" type), this fails because they can't be edited. Since incident.io automatically removes entries from synced types when the source is deleted, Terraform shouldn't try to delete them.

how it works

1. Before attempting to delete a catalog entry, the method fetches the catalog type details.
2. It checks the IsEditable field of the catalog type.
3. If the catalog type is not editable (indicating it's externally synced), the deletion is skipped entirely.
4. For editable catalog types, the deletion proceeds as before.

This ensures that catalog entries for externally synced types (like the built-in "Schedule" type) are not deleted by Terraform, preventing the "Externally synced catalog types can't be edited" error.