test: track TryDeactivateForCollection deactivation race #11424

The Windows .NET 10 BVT job for #11421 failed UnitTests.ActivationsLifeCycleTests.ActivationDataMigrationTests.TryDeactivateForCollection_AtomicallyStartsDeactivation because TryDeactivateForCollection returned Reschedule instead of StartedDeactivation.

Observed output:

Assert.Equal() Failure: Values differ
Expected: StartedDeactivation
Actual:   Reschedule

The assertion failed at test/Orleans.Runtime.Internal.Tests/ActivationsLifeCycleTests/ActivationDataMigrationTests.cs:72 after 138 ms.

Affected run: https://github.com/dotnet/orleans/actions/runs/37592594546
Affected job: https://github.com/dotnet/orleans/actions/runs/37592594546/job/112697404224
Tested head: 143ea0e1e6fc95576d2afd5a83f045d8027c121b from #11421. That PR changes only SiloHealthMonitor cancellation handling and its focused tests, so this activation lifecycle failure is unrelated.

The exact test passed locally on Windows/net10.0 when run in isolation:

Test run summary: Passed!
total: 1
failed: 0
succeeded: 1

Exact test-name searches across open/closed issues and pull requests found no matching tracker or fix. Investigate the race which leaves the activation temporarily in a state requiring rescheduling when the test expects collection deactivation to begin atomically.