Stabilize MCP batch timeout concurrency lease test under full-suite load

[Widthdom]

Description

McpServerTests.ProcessFrameAsync_BatchTimedOutActionRetainsConcurrencyLeaseUntilItDrains_Issue4536_Issue4545 can fail with a NullReferenceException at tests/CodeIndex.Tests/McpServerTests.cs:7158 under the full Release suite on .NET 8.

Observed behavior

• One full Release run passed this test.
• A later full Release run failed only this test on .NET 8 while .NET 9 passed.
• Three immediate isolated .NET 8 reruns all passed (3/3).

This suggests a load-sensitive race or an assertion that dereferences a missing response without first reporting the timed-out state.

Validation command

dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 -p:UseSharedCompilation=false --no-restore --no-build --filter "FullyQualifiedName~ProcessFrameAsync_BatchTimedOutActionRetainsConcurrencyLeaseUntilItDrains_Issue4536_Issue4545"

Related historical work: #4536, #4545.