DI scope disposal (#7205) part 2: scope-resolved services are externally owned, not disposed again by the object tracker #7382

Part 2 of #7205, split from #7328. Its branch issue-7205-di-scope-dispose (head b1a2ba27d2) has commits to reuse.

Scope

When a disposable DI scope is disposed (part 1), the object tracker also disposes the services the scope resolved. So each service is disposed twice, and a root singleton that another source shares is disposed too early. Make the scope the only owner of what it resolves.

• ObjectTracker.MarkExternallyOwned (internal). DependencyInjectionDataSourceAttribute marks what Create returns as externally owned only when the scope is disposable. Document that contract in Create's XML docs.
• ObjectGraphDiscoverer: IAsyncInitializer objects reached through the properties of an externally owned service, at any depth, are still initialized but not disposed by the tracker. Values of TUnit-injected data-source properties stay with the tracker.
• An object counted by another source just before it was marked externally owned is released without being disposed (the race that ExternallyOwnedSingletonTests pins).

Files

src/TUnit.Core/Tracking/ObjectTracker.cs, src/TUnit.Core/Discovery/ObjectGraphDiscoverer.cs, src/TUnit.Core/Attributes/TestData/DependencyInjectionDataSourceSourceAttribute.cs, tests/TUnit.UnitTests/ExternallyOwnedSingletonTests.cs, and the _7205 NonDisposableScopeTests and NestedScopeDependencyTests cases.

Acceptance criteria

• Each scope-owned service is disposed exactly once, in both modes.
• A non-disposable scope's services are still disposed by the tracker.
• Related follow-ups: Let a dependency injection data source say its disposable scope does not own what Create returns #7375 (opt-out), Unit-test external-ownership marking in tracking discovery, including an object reached by both paths #7376 (unit tests for discovery marking).