Avoid blocking Copilot model selection on utility alias resolution

[vritant24]

Summary

• Keep Copilot language model metadata publication from waiting on utility alias endpoint resolution.
• Resolve utility alias overrides in the background and republish when they become available.
• Add regression coverage for stalled alias resolution and for selecting hidden Copilot utility aliases by id.

[Copilot]

Pull request overview

This PR adjusts Copilot language model alias publication so model metadata can be returned without waiting on utility alias override resolution, improving responsiveness of Copilot model selection.

Changes:

• Publishes utility aliases from cached/default endpoints and refreshes override resolution in the background.
• Adds regression coverage for stalled utility alias resolution.
• Adds workbench coverage for selecting hidden Copilot utility aliases by id.

Show a summary per file

File	Description
extensions/copilot/src/extension/conversation/vscode-node/languageModelAccess.ts	Refactors utility alias publication and background override refresh behavior.
extensions/copilot/src/extension/conversation/vscode-node/test/languageModelAccess.test.ts	Adds regression coverage for non-blocking model info publication.
src/vs/workbench/contrib/chat/test/common/languageModels.test.ts	Adds coverage for selecting non-user-selectable Copilot models by id.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 1