Model apps as a set instead of an ordered list (#2)

The arrstack engine returns `apps` alphabetically sorted regardless of the order they were written in, so modelling it as an ordered List makes any config whose order differs fail apply with "Provider produced inconsistent result after apply" and produce perpetual re-diffs. `apps` is semantically a set of app names, so it is now modelled as one.

- Changes the `apps` attribute on `arrstack_secret_backend_role` from `types.List`/`schema.ListAttribute` to `types.Set`/`schema.SetAttribute`
- Reads engine responses back via `types.SetValueFrom`
- Updates unit tests for the set type and adds an order-insensitivity test proving a sorted engine response equals a differently-ordered config value
- No other resources or data sources use the List-of-apps pattern

Reviewed-on: #2
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/provider/resource_secret_backend_role.go

@@ -25,7 +25,7 @@ type secretBackendRoleResource struct {
 type secretBackendRoleModel struct {
 	Backend types.String `tfsdk:"backend"`
 	Name    types.String `tfsdk:"name"`
-	Apps    types.List   `tfsdk:"apps"`
+	Apps    types.Set    `tfsdk:"apps"`
 	TTL     types.Int64  `tfsdk:"ttl"`
 	MaxTTL  types.Int64  `tfsdk:"max_ttl"`
 }
@@ -56,7 +56,7 @@ func (r *secretBackendRoleResource) Schema(_ context.Context, _ resource.SchemaR
 					stringplanmodifier.RequiresReplace(),
 				},
 			},
-			"apps": schema.ListAttribute{
+			"apps": schema.SetAttribute{
 				Description: "arr apps a generated key may access (subset of sonarr, radarr, prowlarr).",
 				ElementType: types.StringType,
 				Required:    true,
@@ -212,9 +212,9 @@ func applyRoleData(m *secretBackendRoleModel, role map[string]interface{}) diag.
 	var diags diag.Diagnostics
 
 	apps := toStringSlice(role["apps"])
-	appList, appDiags := types.ListValueFrom(context.Background(), types.StringType, apps)
+	appSet, appDiags := types.SetValueFrom(context.Background(), types.StringType, apps)
 	diags.Append(appDiags...)
-	m.Apps = appList
+	m.Apps = appSet
 
 	if n, ok := toInt64(role["ttl"]); ok && n != 0 {
 		m.TTL = types.Int64Value(n)