add missing post-write DMA callbacks for AES GCM

[bigbrett]

as described on the tin.

[Copilot]

Pull Request Overview

Adds the missing post-write DMA callbacks for AES-GCM operations to properly finalize client write mappings for output and (when encrypting) the auth tag.

• Invoke post-write processing for the output buffer on success
• Invoke post-write processing for the auth tag on encryption on success

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}