Create service integration tests and run them locally and in CI

## Objective

Cover configuration precedence, invalid options, settings persistence/upgrade/corruption, logging and exception paths, background cancellation, shutdown flushing, and third-party extension registration. Include concurrency and failure-path coverage and run the affected suites locally and in CI.

## 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.

Start by identifying the service integration entry points and affected test suites in the WinForms repository. Review how configuration, persistence, logging, cancellation, shutdown flushing, extensions, concurrency, and failure paths are currently covered. Done means the requested integration tests and documentation are added, and the affected suites pass locally and in CI.

Written by the indexing model from the issue text.