Allow exp of default model

[lramos15]

Allows experimentation of default model via extension

[Copilot]

Pull Request Overview

This PR adds experimentation support for the default language model selection in the GitHub Copilot Chat extension. The change allows A/B testing of different default models through the experimentation service.

• Introduces dependency injection for the experimentation service
• Implements hierarchical default model selection logic with experimentation override
• Restructures the default endpoint selection to prioritize experimental models over system defaults

[jul-stevenson]

@lramos15 do you know how these changes interact with these variables that also change the default model in agent mode? To my understanding, they've only been used for OSWE internal flights. It seems like the intention is the same, so we can switch OSWE to use the chat.defaultLanguageModel variable when they do a next iteration:

vscode.custommodel1
vscode.custommodel1.id
vscode.custommodel1.name

[lramos15]

@lramos15 do you know how these changes interact with these variables that also change the default model in agent mode? To my understanding, they've only been used for OSWE internal flights. It seems like the intention is the same, so we can switch OSWE to use the chat.defaultLanguageModel variable when they do a next iteration:

vscode.custommodel1
vscode.custommodel1.id
vscode.custommodel1.name

@karthiknadig Would know more about these, but I don't think so

[karthiknadig]

@jul-stevenson chat.defaultLanguageModel overrides the one set by vscode.custommodel1.

[jul-stevenson]

Thanks, @karthiknadig! @isidorn and I discussed moving all default model experiments to a single experiment which covers all users. Since chat.defaultLanguageModel overrides vscode.custommodel1 it sounds like that variable won't even work going forward/it can be cleaned up. Does that sound right? Or is vscode.custommodel1 doing something unique that we still need it for?

[karthiknadig]

Only the default model part becomes useless. The other things it does is still useful, like acting as a proxy and allowing you to change visible name of the model in the picker.