fix: exclude non-Active silos from placement

[ReubenBond]

Fixes placement eligibility so compatible silo candidates come from active membership/manifest state by construction. GetCompatibleSilos continues to return SiloAddress[] and uses the cached active-silo list from ISiloStatusOracle.GetActiveSilos() for homogeneous test-mode candidates while preserving deferred placement-filter materialization.

[Copilot]

Pull request overview

This PR updates Orleans placement and cluster-manifest handling so that placement candidates/manifests better reflect current cluster availability (notably around shutdown/termination), while also tightening APIs and reducing manifest duplication.

Changes:

• Update IPlacementContext.GetCompatibleSilos to return ImmutableArray<SiloAddress> and adjust placement directors/tests to match.
• Update PlacementService to use ISiloStatusOracle.GetActiveSilos() for active-silo filtering and filter versioned compatible-silo results by active membership.
• Improve cluster manifest behavior (sync pruning of non-active silos, manifest publishing behavior, manifest deduplication) and add new targeted tests.

Show a summary per file

File	Description
test/Orleans.Core.Tests/Runtime/PlacementServiceTests.cs	Adds coverage for excluding local silo during shutdown; updates test scaffolding for GetActiveSilos()/ImmutableArray.
test/Orleans.Core.Tests/Runtime/LocalPlacementDirectorTests.cs	Updates mocks to return ImmutableArray from GetCompatibleSilos.
test/Orleans.Core.Tests/Runtime/ActivationCountPlacementDirectorTests.cs	Updates mocks to return ImmutableArray from GetCompatibleSilos.
test/Orleans.Core.Tests/Manifest/ClusterManifestProviderTests.cs	New tests validating manifest pruning/publishing behavior and placement’s active-only results.
test/Orleans.Core.Tests/Manifest/ClusterManifestDeduplicationTests.cs	New tests validating structural equality/deduplication of manifest objects.
test/Grains/TestGrains/VersionAwarePlacementDirector.cs	Adjusts to treat compatible silos as IReadOnlyList for ImmutableArray compatibility.
src/Orleans.Runtime/Versions/CachedVersionSelectorManager.cs	Switches cached suitable-silos list to ImmutableArray.
src/Orleans.Runtime/Placement/SiloRoleBasedPlacementDirector.cs	Uses ImmutableArray for compatible-silo list and IsEmpty checks.
src/Orleans.Runtime/Placement/PlacementService.cs	Changes compatible-silo API to ImmutableArray, uses GetActiveSilos(), and filters versioned results by actives.
src/Orleans.Runtime/Placement/ActivationCountPlacementDirector.cs	Updates selector helper signature to accept ImmutableArray.
src/Orleans.Runtime/Manifest/ClusterManifestProvider.cs	Refactors manifest synchronization/publishing and membership-update processing; now depends on IClusterMembershipService.
src/Orleans.Core/Placement/IPlacementDirector.cs	Updates placement hint helper signature to accept ImmutableArray.
src/Orleans.Core/Placement/IPlacementContext.cs	Updates placement context API to return ImmutableArray.
src/Orleans.Core/Manifest/GrainBindings.cs	Small refactor to centralize manifest processing loop into a local function.
src/Orleans.Core/Core/GrainInterfaceTypeToGrainTypeResolver.cs	Refactor to centralize per-manifest cache building logic into a helper method.
src/Orleans.Core.Abstractions/Manifest/GrainProperties.cs	Adds structural equality/hash support for deduplication.
src/Orleans.Core.Abstractions/Manifest/GrainManifest.cs	Adds structural equality/hash support for deduplication.
src/Orleans.Core.Abstractions/Manifest/GrainInterfaceProperties.cs	Adds structural equality/hash support for deduplication.
src/Orleans.Core.Abstractions/Manifest/ClusterManifest.cs	Adds AllGrainManifests ctor and implements manifest/property deduplication.
src/api/Orleans.Core/Orleans.Core.cs	Public API update for GetCompatibleSilos and GetPlacementHint signatures.
src/api/Orleans.Core.Abstractions/Orleans.Core.Abstractions.cs	Public API update for new ClusterManifest ctor and new equality members.

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 6

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/Orleans.Runtime/MembershipService/SiloStatusOracle.cs:59

• GetActiveSilos returns the internal cached array instance. Since arrays are mutable, a caller can inadvertently (or intentionally) modify the returned array and corrupt the oracle’s cache, affecting placement/liveness decisions.

        public SiloAddress[] GetActiveSilos()
        {
            EnsureFreshCache();
            return _activeSilos;
        }

• Files reviewed: 14/14 changed files
• Comments generated: 6