Python: [Feature]: Cancellation request for agent generation loop

### Description

# Scope
We are looking for a simple way to stop the generation loop mid-execution. A lot of agentic applications support this behavior (e.g., Microsoft Copilot), but there is no simple way of actually doing this with Microsoft Agent Framework.

# Why is this useful
- Stopping long lasting/expensive/... generations (User might have sent the wrong query)

# Our current custom implementation
Currently we iterate over a ResponseStream and check whether a variable `is_canceled` was set. If that is the case, we break from the ResponseStream loop.

**Why this is not as easy as it sounds**
When working with (custom) HistoryProvider instances, we are required to implement a before_run method that "repairs" the conversation history/context in some cases -> E.g. when the last element is a function_call element, we need to delete it, as continuing without any result is not possible right now

### Code Sample

```markdown

```

### Language/SDK

Python