[Feature]: Allow saving partial AgentSession state when streaming is cancelled

@rogerbarreto is already working on this.

Since Sep 8, 2026.

### Description

When using `AIAgent.RunStreamingAsync` with an `AgentSession`, cancelling the streaming operation via `CancellationToken` prevents the current interaction from being persisted into the session.

This is expected from a cancellation-flow perspective, but it creates an issue for chat applications: the user may have already received a partial assistant response, while the `AgentSession` does not contain either the user message or the partial assistant response.

As a result, the visible conversation shown to the user can diverge from the conversation state stored in the `AgentSession`.

It would be useful to have an opt-in flag/API that allows persisting the interaction up to the point where cancellation occurred.

## Why this matters

In real-time chat UIs, cancellation does not always mean "discard the turn". Often it means "stop generating now, but keep what the user already saw."

Without a supported way to persist that partial turn, applications have to choose between:

1. losing conversation consistency between UI and session state, or
2. implementing custom workarounds outside the Agent Framework session model.

An explicit opt-in flag would make this behavior predictable and avoid surprising session-history gaps.

### Language/SDK

Both