test(workbench): session tests leave created files (Untitled*.ipynb) behind after cleanup

What happens: The Workbench session-cleanup fixture quits/removes the session, but files created inside a session during a test — most visibly JupyterLab Untitled*.ipynb notebooks from test_launch_jupyter / test_jupyterlab_extensions — remain in the test account's home directory. Over many runs these accumulate (16+ observed on a live deployment).

Why it matters: Test litter accrues in the shared test account, and stale leftover notebooks can pop dialogs (e.g. a lingering "Error Starting Kernel" modal) that intercept clicks in later runs — a latent source of flakiness.

Scope: Non-destructive cleanup of VIP-created files at session/test teardown (tag or track what the test creates and remove only that; must never touch pre-existing user content). Distinct from session cleanup (#467), which is about quitting sessions, not deleting files.

Not a blocker for #484 — surfaced during its live validation.

Locate the Workbench session-cleanup fixture and inspect test_launch_jupyter and test_jupyterlab_extensions, where the leftover files are created. First trace how session teardown currently works and how VIP-created files could be identified without including pre-existing content. Done means teardown removes only files created during the test and the affected tests no longer leave Untitled*.ipynb files behind.