Expose contextSizeOptions for BYOK models

Built-in Copilot models support a configurable context size in the thinking effort selector.
BYOK models do not - the only workaround is to create separate BYOK model definitions with differing contextWindow size options - and care must be taken when duplicating model IDs.
Feature request is to add a `contextSizeOptions` parameter or similar to the BYOKModelCapabilities interface used in chatLanguageModels.json to allow using the built-in selector instead.