Add prepare_model_settings hook

[dsfaccini]

• Closes #

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• Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
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Pre-Merge Checklist

• New tests for any fix or new behavior, maintaining 100% coverage.
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[DouweM]

@dsfaccini Btw I'm thinking that instead of making this a prepare hook, it makes more sense to have model_settings on Agent and agent.run accept a callable that receives the runcontext, as an alternative to a plain ModelSettings dict. That's consistent with the other things that are dynamic per run step, like instructions and metadata (both arguments take callables too). We only really need a prepare hook for tools, as that doesn't dynamically determine the result, but dynamically modify the already-registered tools.