Create automated lifetime and shutdown coverage and run it in CI

## Objective

Add focused unit and integration coverage for startup forms, custom application contexts, startup failures, cancellation, host-initiated shutdown, form-initiated shutdown, exception paths, and repeated runs where supported. Run locally and in the normal CI matrix; avoid timing-fragile tests.

## Completion guidance

- Keep the implementation focused on the objective while allowing the assignee to improve the approach based on research.
- Document important decisions, rejected alternatives, dependencies, and follow-up work.
- Add or update tests and documentation appropriate to the work.
- Verify locally and in CI where the task changes code or generated artifacts.

No files or existing tests are named; start by locating the WinForms startup, application-context, lifetime, and shutdown tests, then inspect the normal CI matrix. Done means focused local and CI coverage for the listed startup, cancellation, shutdown, exception, and repeated-run paths without timing-fragile tests, with key decisions and follow-up work documented.

Written by the indexing model from the issue text.