add support for custom anthropic models

[udit-329]

What it does

Adds support for custom anthropic models. This works similar to how user can configure custom openAI models for situations such as when the model is hosted on a custom url.

Addresses #16666

How to test

If a user has an Anthropic model that is accessed from a custom endpoint, they can add it in settings.json similar to how it can already be done for openAI models.

"ai-features.AnthropicCustom.customAnthropicModels": [
        {
            "model": "claude-sonnet-4",
            "url": "<custom-url>",
            "id": "<model-ID>",
            "apiKey": "<API-Key>"
        }
    ]

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

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[sdirix]

Thanks for the contribution ❤️

I tested it successfully by defining a custom model against the official API endpoint 👍

However there is an issue with the "apiKey"-less path, see my comments.

[ndoschek]

Hi @udit-329, just a gentle ping, the review is done and there are a few points that would need your input before we can proceed. TIA!

[sdirix]

Works great for me. Thanks! ❤️