fix(provider): mark DeepSeek models as BYOK

[Vizards]

Summary

• Mark DeepSeek language model metadata with isBYOK: true so new VS Code BYOK-aware logic classifies the provider correctly.
• Keep the local non-public metadata typing in sync with the runtime shape.

Why

VS Code now distinguishes BYOK models from Copilot/CAPI models using LanguageModelChatInformation.isBYOK. DeepSeek requests are served with the user supplied API key, so the provider should opt into that signal.

Upstream VS Code PR: microsoft/vscode#321090

Validation

• npm run format:check
• npm run compile

[Copilot]

Pull request overview

This PR updates the DeepSeek chat model metadata returned to VS Code so the models are explicitly classified as BYOK (Bring Your Own Key) via LanguageModelChatInformation.isBYOK, aligning with new VS Code logic that differentiates BYOK from Copilot/CAPI-backed models.

Changes:

• Extend the local ModelPickerChatInformation metadata type to include isBYOK.
• Set isBYOK: true on the model chat info objects returned by toChatInfo.

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