Make rewind session upon cancellation accessible via API server

Start in api_server.py and compare the existing API behavior with the cancellation and rewind flow exposed by the Debug Web UI. Investigate the proposed runner.rewind_async entry point and determine how it should be exposed through an API endpoint. Done means the API server can cancel generation and rewind the session to its previous state without requiring clients to save, delete, and recreate sessions.

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🔴 Required Information

Please ensure all items in this section are completed to allow for efficient
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If an item is not applicable to you - please mark it as N/A

Is your feature request related to a specific problem?

Agents may sometimes exhibit a heavy tool use, resulting in long processing times. In such a situation, one often wants to abort the generation, return to the last state, and rephrase the query. When interacting with our agents, we heavily rely on the ADK API (adk api_server), which doesn't seem to support a clean way to abort a generation and rewind the session to the previous state.

Describe the Solution You'd Like

The Debug Web UI (adk web) provides a button that allows to cancel the generation and rewind the session to the previous state. An analogous feature for the ADK API (adk api_server) is needed.

Impact on your work

Since we are currently not able to abort generation in a clean way, users often get frustrated when the agents take a wrong path and get lost in the various tools.

Willingness to contribute

N/A

🟡 Recommended Information

Describe Alternatives You've Considered

The only alternative we see with the ADK API so far would be save the session before every turn, delete it upon cancellation, and regenerate the session with the previous copy (create with id), but this seems overly excessive, especially since the ADK Web UI already has the functionality we are looking for.

Proposed API / Implementation

I'd expect that something like runner.rewind_async would be present in the api_server.py and accessible through one of the endpoints.

Additional Context

N/A