refactor: extract domain models from provider into internal/model

- Move Server/Domain/DNSRecord/Bucket/StorageInstance/CDNInstance/Order/Error structs to internal/model/
- Provider interfaces now reference model.* types
- Rainyun provider implementations import model package for data types
- CLI commands updated to use model.DNSRecord directly

Separates pure data structures from service contract interfaces,
following hexagonal architecture: models (entities) live independently
from ports (provider interfaces) and adapters (rainyun/* implementations).

internal/model/errors.go

@@ -0,0 +1,18 @@
+package model
+
+import "fmt"
+
+type Error struct {
+	Code    int
+	Message string
+	Detail  string
+	Raw     []byte
+}
+
+func (e *Error) Error() string {
+	return e.Message
+}
+
+func (e *Error) Debug() string {
+	return fmt.Sprintf("%s\n%s", e.Detail, string(e.Raw))
+}