Stop PlacementService workers during shutdown

[ReubenBond]

Fixes #9827.

Why

PlacementService starts background worker loops but did not participate in the silo lifecycle. During shutdown those workers could continue into placement resolution and touch keyed services after the silo was already stopping, which could surface the logged ObjectDisposedException from IServiceProvider disposal.

What changed

• register PlacementService as an ISiloLifecycle participant and stop it at ServiceLifecycleStage.RuntimeInitialize + 1
• signal and await the placement workers so they do not outlive shutdown
• reject new queued placement work once shutdown begins, while keeping the stop checks only at the points which actually protect against post-stop service resolution
• add focused regression tests covering worker shutdown and post-stop placement rejection

Notes

This fixes the lifetime mismatch directly instead of catching and suppressing ObjectDisposedException in the worker loop.

Validation

• dotnet build Orleans.slnx -nologo
• dotnet test test\\Orleans.Core.Tests\\Orleans.Core.Tests.csproj -nologo --filter "FullyQualifiedName~PlacementServiceTests"

Microsoft Reviewers: Open in CodeFlow