Fix activation-count placement fallback for partial statistics

[ReubenBond]

Summary

• avoid per-placement HashSet allocation in ActivationCountPlacementDirector
• use direct random compatible-silo fallback when no compatible silo stats are available
• preserve overload rejection when all compatible silos are known and overloaded

Validation

• dotnet test .\\test\\TesterInternal\\TesterInternal.csproj --filter "FullyQualifiedName~UnitTests.General.ElasticPlacementTests.LoadAwareGrainShouldNotAttemptToCreateActivationsOnOverloadedSilo" --nologo --verbosity minimal
• repeated flaky-case validation: 5x --no-build runs of the same test

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR improves the activation count-based placement strategy by optimizing performance and fixing edge cases in fallback logic when silo statistics are partially available.

Changes:

• Removed per-placement HashSet allocation by eliminating the ToSet() call on compatible silos
• Implemented reservoir sampling for uniform random selection when silos lack statistics
• Changed empty-cache fallback from local silo to random compatible silo placement

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

src/Orleans.Runtime/Placement/ActivationCountPlacementDirector.cs:112

• The error message mentions "from {silos.Length} compatible silos" but then enumerates all silos from _localCache (which includes incompatible silos). This could be confusing when debugging. Consider either:

1. Filtering the enumeration to only show compatible silos: Utils.EnumerableToString(silos.Select(s => KeyValuePair.Create(s, _localCache.GetValueOrDefault(s))), ...)
2. Clarifying the message: "Unable to select a candidate from {silos.Length} compatible silos (all are overloaded). All silo stats: {Utils.EnumerableToString(all, ...)}"

            throw new SiloUnavailableException($"Unable to select a candidate from {silos.Length} compatible silos: {Utils.EnumerableToString(all, kvp => $"SiloAddress = {kvp.Key} -> {kvp.Value}")}");