Add the option to reduce context window

[ryx2]

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With the https://github.com/All-Hands-AI/OpenHands/pull/4977 change being in 0.14, now we should allow users to set a lower context window than their LLMs maximum context window size. This allows the code to perform better if the model's performance degrades too significantly if you use the maximum context window, you can use this to set a shorter window, manually.

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