A new enum attribute must not default to unspecified

Trigger: adding an enum to a resource that already has released versions, or a Provider migration (...) job failing expected NoOp, got action(s): [update] while every create-and-apply test passes.

Fix: three parts, all needed.

Optional: true, Computed: true,                                                        // 1. no Default
PlanModifiers: []planmodifier.String{stringplanmodifier.UseNonNullStateForUnknown()},  // 2.
// 3. flatten an unmapped value (the enum's zero value "") to utils.UNSPECIFIED, not ""

A default claims what the API stores for an omitted field, and it usually stores something else. Part 3 stops flatten writing "", which the schema disallows. Part 2 stops the framework planning an omitted nested Optional+Computed attribute as unknown; the non-null variant keeps a list element added on update unknown. Removing the attribute now keeps the previous value, so name unspecified as the reset in the description.

Why it hides: sending a value means the API echoes it, so the map never misses and the plan is never unknown. Only migration and PlanOnly steps omit the field.