Add directory migration join regression

[ReubenBond]

Summary

Adds a regression test for a local-to-distributed directory migration join case where a joining silo must not leave stale directory entries on the previous owner.

Changes

• Add JoiningSilo_DoesNotLeaveStaleEntriesOnPreviousOwner coverage.
• Keep the existing ElasticChaos stress test separate from the deterministic migration regression.

Stack notes

This branch is stacked above #10053. The focused review range is:

fix/directory-transfer-payload-batching..fix/directory-migration-regression-test

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.

[Copilot]

Pull request overview

Adds a targeted regression test to harden Orleans distributed grain directory resilience when a new silo joins and triggers range migration under concurrent directory activity.

Changes:

• Refactors directory partition integrity verification into a shared helper (CheckIntegrityAsync).
• Adds a new stress/regression test which runs concurrent directory load while starting an additional silo and repeatedly validating partition integrity.
• Minor formatting cleanup in existing chaos test.

Show a summary per file

File	Description
test/Orleans.GrainDirectory.Tests/GrainDirectory/GrainDirectoryResilienceTests.cs	Adds a joining-silo migration regression test and centralizes partition integrity checks into a helper.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 2

[ReubenBond]

Rebuilt on top of the updated #10050 head.

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

test/Orleans.GrainDirectory.Tests/GrainDirectory/GrainDirectoryResilienceTests.cs:298

• Same issue as above: currentRange.Difference(previousRange) can return more than one segment. SingleOrDefault() will throw if the added portion splits into multiple ranges. Iterate over the full Difference(...) result set and create an expected "acquire" operation per segment.

                var addedRange = currentRange.Difference(previousRange).SingleOrDefault();
                if (!addedRange.IsEmpty)
                {

• Files reviewed: 3/3 changed files
• Comments generated: 1