Add extension contract tests

[zackverham]

Summary

• Adds test/extension-contract-tests/ — a contract test suite that validates which VSCode and Positron APIs the extension calls, with what arguments, and what it expects back
• Tests mock the vscode and positron modules, import actual extension source code against those mocks, and assert API interactions
• Includes compile-time conformance checks (tsc) that verify mock properties exist in the real @types/vscode and Positron type definitions
• No Go binary, no HTTP server, no network — tests run entirely in-process using Vitest with module aliasing
• Adds just test-extension-contracts and just check-extension-contract-conformance recipes
• Adds extension-contract-tests workflow to CI (pull request, main, nightly)

What's tested

Test File	Extension Source	APIs Validated
activation	extension.ts	activate() wiring: trust, URI handler, commands, contexts
auth-provider	authProvider.ts	authentication.registerAuthenticationProvider
connect-filesystem	connect_content_fs.ts	workspace.registerFileSystemProvider, FileSystemError
dialogs	dialogs.ts	window.showInformationMessage (modal), l10n.t
document-tracker	entrypointTracker.ts	Editor/document change events, commands.executeCommand("setContext")
extension-settings	extension.ts	workspace.getConfiguration("positPublisher")
file-watchers	watchers.ts	workspace.createFileSystemWatcher, RelativePattern
interpreter-discovery	utils/vscode.ts	commands.executeCommand, workspace.getConfiguration, Positron runtime
llm-tools	llm/index.ts	lm.registerTool
open-connect	open_connect.ts	window.showInputBox, workspace.updateWorkspaceFolders
positron-settings	utils/positronSettings.ts	workspace.getConfiguration("positron.r")
window-utils	utils/window.ts	window.showErrorMessage, withProgress, createTerminal

Test plan

• just test-extension-contracts passes
• just check-extension-contract-conformance passes
• CI extension-contract-tests workflow passes

🤖 Generated with Claude Code