Fix flake in TestRegistryLifecycle pnpm-style symlink test

[jakebailey]

Noticed this test flake: https://github.com/microsoft/typescript-go/actions/runs/25234356902/job/73997098962

--- FAIL: TestRegistryLifecycle (0.01s)
    --- FAIL: TestRegistryLifecycle/pnpm-style_symlinks_only_grant_granular_updates_to_workspace_packages (3.78s)
        registry_test.go:641: assertion failed: false (bool) != true (true bool): bucket should be dirty after registry package change
FAIL
FAIL	github.com/microsoft/typescript-go/internal/ls/autoimport	4.532s

[Copilot]

Pull request overview

Fixes a flaky auto-import registry lifecycle test by ensuring assertions read cache stats from the same request-time snapshot that observed the file change, avoiding races with background auto-import warming that can advance the session snapshot.

Changes:

• Switch the pnpm-style symlink test to read AutoImportRegistry cache stats from the snapshot provided by Session.WithLanguageServiceAndSnapshot instead of session.Snapshot().
• Add the needed internal/ls import to type the callback parameter.