make copilot-fast callable from api

[sbatten]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces a centralized ModelAliasRegistry to manage model aliases and refactors the hardcoded copilot-fast to gpt-4o-mini mapping. The registry provides a reusable mechanism for defining model aliases and resolving them across the codebase.

Key Changes

• Created a new ModelAliasRegistry singleton class for managing model aliases bidirectionally
• Refactored ModelMetadataFetcher.getChatModelFromFamily() to resolve aliases before family lookup, removing the hardcoded copilot-fast condition
• Updated LanguageModelAccess to register alias model entries and resolve aliases when finding endpoints

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/platform/endpoint/common/modelAliasRegistry.ts	New singleton registry for bidirectional model alias management with registration and resolution methods
src/platform/endpoint/node/modelMetadataFetcher.ts	Removed hardcoded copilot-fast mapping, now uses registry to resolve aliases before family lookup
src/extension/conversation/vscode-node/languageModelAccess.ts	Registers alias model entries in language model info list and resolves aliases when finding endpoints

[Copilot]

Setting family: alias on line 212 may introduce confusion since the alias (e.g., 'copilot-fast') would be used as the family instead of the resolved model's actual family (e.g., 'gpt-4o-mini'). Consider whether the family should remain consistent with the original model's family to maintain proper categorization.

Suggested change

	family: alias,
	family: model.family,

[sbatten]

@lramos15 If our internal getChatEndpoint took id instead of family, which it effectively does, I'd follow this suggestion as you also brought it up.