Add distributed remote grain directory compatibility

[ReubenBond]

Summary

Adds the distributed remote grain directory compatibility path on top of #10047 so distributed-directory silos can serve legacy IRemoteGrainDirectory requests during rolling upgrades from the local grain directory.

Changes

• Add DistributedRemoteGrainDirectory backed by DistributedGrainDirectory.
• Register compatibility system targets for the directory service and cache validator types.
• Route remote register, lookup, and unregister calls through the distributed directory implementation.
• Add mixed local/distributed rolling-upgrade compatibility coverage and resilience fixes.

Stack notes

This branch is rebased on top of #10047. The focused review range is:

fix/directory-snapshot-transfer-ranges..feature/distributed-remote-grain-directory

Follow-on PRs:

• Batch large directory transfer payloads #10053 stacks above this branch for large transfer payload batching.
• Add directory migration join regression #10049 stacks above Batch large directory transfer payloads #10053 for the directory migration join regression.

Validation

Focused validation run locally:

• dotnet test test\Orleans.GrainDirectory.Tests\Orleans.GrainDirectory.Tests.csproj --framework net10.0 --filter "FullyQualifiedName~GrainDirectoryRollingUpgradeTests|FullyQualifiedName~GrainDirectoryPartitionBatchingTests|FullyQualifiedName~GrainDirectoryResilienceTests.JoiningSilo_DoesNotLeaveStaleEntriesOnPreviousOwner" -- -parallel none -noshadow

GrainDirectoryResilienceTests.ElasticChaos was also attempted in a broader run and timed out; the deterministic non-chaos coverage above passed.

[ReubenBond]

Rebuilt on top of #10053. The base compatibility pieces needed for earlier CI have moved into #10047; this PR now carries the follow-up distributed remote directory resilience and mixed-upgrade fixes.

[Copilot]

Pull request overview

This PR adds a compatibility layer so silos running the new DistributedGrainDirectory can continue to serve legacy IRemoteGrainDirectory requests during rolling upgrades from LocalGrainDirectory, alongside testing-host and test-suite updates to validate mixed-mode upgrade scenarios.

Changes:

• Add DistributedRemoteGrainDirectory system targets backed by DistributedGrainDirectory to handle legacy remote directory calls during rolling upgrades.
• Add local-directory compatibility system targets (IGrainDirectoryClient/IGrainDirectoryPartition) to enable mixed local/distributed interop.
• Extend testing infrastructure and add/adjust tests to cover rolling upgrade and resilience behavior.

Show a summary per file

File	Description
test/Orleans.GrainDirectory.Tests/GrainDirectory/GrainDirectoryRollingUpgradeTests.cs	New rolling-upgrade test exercising mixed Local→Distributed directory transitions with diagnostic log capture.
test/Orleans.GrainDirectory.Tests/GrainDirectory/GrainDirectoryResilienceTests.cs	Use runtime PartitionsPerSilo from DirectoryMembershipService for integrity checks.
src/Orleans.TestingHost/TestCluster.cs	Initialize client gateways from active silo gateway endpoints (fallback to configured ports).
src/Orleans.TestingHost/InProcTestClusterOptions.cs	Add UseTestClusterGrainDirectory option (internal) to allow opting out of the test grain directory.
src/Orleans.TestingHost/InProcTestClusterBuilder.cs	Default UseTestClusterGrainDirectory to true for backwards-compatible behavior.
src/Orleans.TestingHost/InProcTestCluster.cs	Conditionally register the test grain directory based on UseTestClusterGrainDirectory.
src/Orleans.Runtime/Hosting/CoreHostingExtensions.cs	Update distributed directory registration to construct DirectoryMembershipService with explicit partitioning/boundaries configuration.
src/Orleans.Runtime/GrainDirectory/RemoteGrainDirectory.cs	Add optional registerAsSystemTarget to avoid system target ID conflicts when distributed directory is active.
src/Orleans.Runtime/GrainDirectory/LocalGrainDirectoryCompatibility.cs	New compatibility system targets enabling distributed-directory silos to interact with local-directory silos.
src/Orleans.Runtime/GrainDirectory/LocalGrainDirectory.cs	Avoid registering local IRemoteGrainDirectory targets when distributed directory is active; register compatibility targets for local-mode silos.
src/Orleans.Runtime/GrainDirectory/GrainDirectoryHandoffManager.cs	Add additional rolling-upgrade diagnostic logging around split-partition handoff and removal.
src/Orleans.Runtime/GrainDirectory/DistributedRemoteGrainDirectory.cs	New distributed-backed IRemoteGrainDirectory implementation with batching, retries, and duplicate-activation cleanup.
src/Orleans.Runtime/GrainDirectory/DistributedGrainDirectory.cs	Use configured partitions-per-silo and register distributed-backed IRemoteGrainDirectory targets for upgrade compatibility.
src/Orleans.Runtime/GrainDirectory/DirectoryMembershipSnapshot.cs	Add default ring-boundary function matching LocalGrainDirectory and use it consistently (including for Default).
src/Orleans.Runtime/GrainDirectory/DirectoryMembershipService.cs	Parameterize membership snapshot generation by partitions-per-silo and boundary function.

Copilot's findings

• Files reviewed: 15/15 changed files
• Comments generated: 2