Context length cannot be changed from the model picker #599

Problem

When choosing a model in an ACP client, the context length cannot be changed. The model picker shows the model and reasoning effort, but there is no context-length control.

This happens with @agentclientprotocol/codex-acp 2.1.1 in JetBrains Rider 2026.2.

Why this should change

The context length decides how much of the conversation the model can use. Users need to choose it while selecting a model, the same way they choose the model and reasoning effort. ACP already lets an agent expose this next to the model selector. Without it, the window size stays fixed to whatever the selected model uses, and the user only learns the size after a turn has started.

Expected result

The model picker includes a context-length control. Choosing a value applies it to the current session and to later turns. If a model supports only one length, that length is shown and selected.