Fix enableDevContainer test output channel mock

[lambrianmsft]

Summary

• Fix the enableDevContainerIntegration.test.ts output channel mock to include appendLog, matching the shared IAzExtOutputChannel test shape.
• Clear ext.designTimeInstances before and after each integration test to avoid stale design-time state leaking across tests.
• Addresses CI-only Vitest unhandled rejections from the Linux design-time validation path calling ext.outputChannel.appendLog.

Validation

• npx biome check --write apps\vs-code-designer\src\app\commands\enableDevContainer\__test__\enableDevContainerIntegration.test.ts
• npx vitest run src\app\commands\enableDevContainer\__test__\enableDevContainerIntegration.test.ts
• npx vitest run src\app\utils\codeless\__test__\startDesignTimeApi.test.ts
• npx vitest run src\app\commands\enableDevContainer\__test__\enableDevContainerIntegration.test.ts src\app\utils\codeless\__test__\startDesignTimeApi.test.ts
• pnpm run test:extension-unit