[Immutable Models M2] messages.BlockResponse: message/internal split + validation

[jordanschalm]

Summary
Implement message/internal type split and explicit validation for messages.BlockResponse.
For full context & examples, see the epic: #7694

Definition of Done

• Network model lives in messages (typedef to flow.Untrusted... where applicable)
• Implement UntrustedMessage interface and ToInternal() to convert to internal model
• If used internally, add validated flow.BlockResponse equivalent and convert before use