Unit-test external-ownership marking in tracking discovery, including an object reached by both paths #7376

Problem

PR #7328 (fixing #7205) gave ObjectGraphDiscoverer's tracking discovery a heldByExternalOwner flag, an injectedValues set, and a second traversal, so IAsyncInitializer objects beneath a scope-owned service are marked externally owned while values of TUnit-injected data-source properties stay with the tracker.

Coverage today is integration-level, in tests/TUnit.TestProject/Bugs/_7205:

• an IAsyncInitializer dependency beneath a scope-owned service (NestedScopeDependencyTests), disposed exactly once;
• a TUnit-injected IAsyncInitializer beneath a scope-owned service (NestedProbeService.Fixture), disposed exactly once.

Not covered: one object reached by both paths in the same graph, once as an injected property value and once as an IAsyncInitializer property of a scope-owned service. Nothing pins which owner wins.

Raised by the Claude review on #7328 (head ca412cb, item 4).

Acceptance criteria

• Unit tests in tests/TUnit.UnitTests cover the three cases above directly against the discoverer, including the shared-object case, in source-generated and reflection property registries.
• The shared-object case asserts that the tracker keeps an object that a TUnit-injected property supplies, and disposes it exactly once.

Blocked by #7382 (the ownership portion of replacement PR #7384). PR #7328 was closed without merging. Its implementation and review remain historical context.