Add smoke test for installed source wired to real AppAgentHostApplicator

[curtisman]

Summary

Add a thin smoke/integration test that wires the installed-agent source lifecycle to the real AppAgentHostApplicator.

Background

The source lifecycle/barrier tests use faithful fake hosts, and the single-lock/no-interleave invariant is pinned in appAgentHost.spec.ts. What is missing is a small test that composes the real installed source path with the real dispatcher applicator, so a future regression from coordinated replaceProvider back to naive remove+add would fail outside the isolated unit test.

Suggested Coverage

• install/update/uninstall through createDefaultInstalledAgentSource
• a real AppAgentHostApplicator using a real command lock
• assertion that replacement runs under the intended single-lock section and does not expose old+new coexistence

Source

Deferred from PR #2611 review-log cleanup.