Keep the first user message by default in condensers

[enyst]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

---

Give a summary of what the PR does, explaining any non-trivial design decisions

This PR proposes to set the default keep_first to 1 everywhere in condensers, in a best effort attempt to preserve the first user message. We already did this in one of the condensers, this does it in all.

The limit is configurable and accepts 0, but the use cases for 0 seem maybe eval cases or specific cases, while the first user message is important also because currently, we hook prompt extensions into it.

---

Link of any specific issues this addresses

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:b6391d4-nikolaik   --name openhands-app-b6391d4   docker.all-hands.dev/all-hands-ai/openhands:b6391d4