[TPI] BYOK suport in Agent Host

[vritant24]

Refs: #322977

• Any OS @pierceboggan
• Any OS @aeschli

Complexity: 3

Authors: @vritant24

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Setup

• set chat.agentHost.byokModels.enabled to `true
• restart VS Code
• Ensure Copilot Agent Host is being used for chat

For adding Built-in BYOK providers:

get a secret (apices) from this key vault (I recommend gemini or open_ai)
Using Chat: Manage Language Models > Add Models, add the corresponding provider (Gemini or Open AI)
the UI should take you through the configuration
For adding 3rd party extension based providers, there can be many ways, but I have tried Ollama, and HuggingFace.

Testing

With the Setting on:

Note: Visibility toggles in Manage Models View will not apply in the Agent Host model picker at this point.

• The model picker should show the BYOK models in groups in the Editor Window
• You should be able to select a BYOK model and start a chat with it
• The chat should be reflected in all client windows (Agent Window and Editor Window)

With the setting off (needs VS Code restart):

• The model picker should not show BYOK models.
• All existing GitHub Copilot models should work with no issue.

[aeschli]

@vritant24 Opening the key vault page it says: 'You are unauthorized to view these contents.'

[aeschli]

I tested with a Google API key and an Ollma model using the Ollama extension.