Components.AI: Represent stopped and incomplete answers explicitly

### Is your feature request related to a problem? Please describe the problem.

A cancelled or failed streaming response can appear indistinguishable from a completed answer.

### Describe the solution you'd like

Keep partial content, mark the message as incomplete with the correct stopped/failed outcome, retain message actions, and expose retry at turn scope.

### Additional context

Leaf task for #68227 requirements F12 and G8.

See: https://github.com/dotnet/aspnetcore/pull/68325#discussion_r3795899991.

Start by reading the Components.AI requirements in #68227, especially F12 and G8, then review the linked discussion on pull request #68325. Done means cancelled or failed streaming responses preserve partial content, expose the correct stopped or failed outcome, retain message actions, and provide retry at turn scope.

Written by the indexing model from the issue text.