Resolves the test failure in CanShutdownServerProcess when setting capacity for the _nodeContexts dictionary #12187

Fixes #12187

Context

When attempting to set capacity for _nodeContexts dictionary to improve performance, the test MSBuildServer_Tests.CanShutdownServerProcess(byBuildManager: True) failed due to NullReferenceException when accessing host.BuildParameters.MaxNodeCount without null-checking.

Changes Made

• Added null-safe check for host.BuildParameters before accessing MaxNodeCount
• Validate MaxNodeCount > 0 to avoid invalid capacity values
• Use Environment.ProcessorCount for concurrencyLevel to optimize multi-threaded scenarios
• Provide fallback initialization when BuildParameters is null or invalid

Testing

• Verified MSBuildServer_Tests.CanShutdownServerProcess passes with both byBuildManager: True and False
• Confirmed the fix enables capacity optimization while maintaining thread-safety

Notes

This change allows performance optimization through pre-allocated dictionary capacity while ensuring test stability and proper null handling.