feat(vscode): startup extension on external models files

[benfdking]

• before if you were to open vscode and an external models file, the extension would not start automatically

[Copilot]

Pull Request Overview

This PR adds support for automatically starting the VS Code extension when external models files (external_models.yaml or external_models.yml) are opened, addressing an issue where the extension wouldn't activate automatically for these file types.

• Extends the Language Server Protocol (LSP) document selector to include external_models.yaml and external_models.yml files
• Adds YAML language activation event to trigger extension startup
• Includes comprehensive test coverage for both YAML file extensions

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
vscode/extension/src/lsp/lsp.ts	Extends LSP document selector to include external_models.yaml and external_models.yml patterns
vscode/extension/package.json	Adds "onLanguage:yaml" activation event to automatically start extension for YAML files
vscode/extension/tests/external_models.spec.ts	New test file verifying extension startup functionality for both .yaml and .yml external models files