perf(runtime): lazily allocate activation request tracking

[ReubenBond]

Summary

• Lazily allocate activation waiting/running request collections instead of creating them for every activation.
• Return empty request tracking collections for reuse.
• Preserve null-safe inactivity and request-count semantics for activations with no queued or running requests.

Validation

• dotnet build src\Orleans.Runtime\Orleans.Runtime.csproj --nologo --verbosity minimal -property:UseSharedCompilation=false
• dotnet build test\Grains\TestVersionGrains\TestVersionGrains.csproj --nologo --verbosity minimal -property:UseSharedCompilation=false
• dotnet build test\Grains\TestVersionGrains2\TestVersionGrains2.csproj --nologo --verbosity minimal -property:UseSharedCompilation=false
• dotnet test test\Orleans.Runtime.Tests\Orleans.Runtime.Tests.csproj --no-build --nologo --verbosity minimal -property:UseSharedCompilation=false

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR reduces per-activation allocations in the Orleans runtime by lazily creating (and reusing) the collections used to track queued and running requests for an activation, while keeping existing “no requests” semantics intact.

Changes:

• Lazily allocate _waitingRequests and _runningRequests in ActivationData and update related logic to be null-safe.
• Introduce shared pools for the request-tracking collections and return them for reuse when they become empty.

Show a summary per file

File	Description
src/Orleans.Runtime/Catalog/GrainTypeSharedContext.cs	Adds shared pools plus rent/return helpers for request-tracking collections.
src/Orleans.Runtime/Catalog/ActivationData.cs	Switches request tracking to lazy allocation, adds return-to-pool logic, and updates null-safe request-count/inactivity checks.

Copilot's findings

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