test(09-05): add failing tests for writable fill and validate

- Create and update must Fill then Validate and return D-10 422 field errors
- Schema bindings exclude protected, cased, nested, and unknown keys
- Missing Fill or Validate capability fails closed with controller context

cabana/contracts.go

@@ -56,12 +56,20 @@ func (BackendUser) TableName() string { return "backend_users" }
 // Option is a dropdown choice shared with later schema plans.
 type Option = pact.Option
 
+// WritableField is one schema field bound to a model fill key at activation.
+// FillKey comes from the model column, not from request JSON.
+type WritableField struct {
+	Name    string
+	FillKey string
+}
+
 // CompiledController is one admin controller after YAML compilation.
 type CompiledController struct {
 	PluginID   string
 	Controller pact.AdminController
 	List       *ListSchema
 	Form       *FormSchema
+	Writable   []WritableField
 }
 
 // Registry is the immutable controller map keyed by controller ID.