Look for apphost when considering node reuse

[rainersigwald]

There was a collision between #13220 (at end of build, look for other
running MSBuild processes and decide whether node reuse is likely to be
worthwhile) and #13175 (use an apphost on core). This broke the
overprovisioning detection, which was looking for dotnet instead
of the new MSBuild.

Always search for MSBuild instead, and remove the now-unnecesary
"try to filter dotnet.exe processes to those running MSBuild.dll
code too.

[Copilot]

Pull request overview

Adjusts system-wide node detection to account for the MSBuild apphost so over-provisioning detection/node reuse decisions look for MSBuild processes instead of dotnet.

Changes:

• Switch expected process name for node enumeration to MSBuild.
• Remove dotnet-specific command-line filtering intended to detect MSBuild.dll under dotnet.

Comments suppressed due to low confidence (2)

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs:644

• There are unit tests for the over-provisioning logic (NodeProviderOutOfProc_Tests), but they override CountSystemWideActiveNodes, so this GetProcessesByName-based enumeration path isn't covered. Consider adding a test that would fail if an extension (".exe") is accidentally passed into GetProcessesByName on Windows (e.g., by asserting the computed expected process name is extensionless).

            var expectedProcessName = Constants.MSBuildExecutableName;

            Process[] processes;
            try
            {
                processes = Process.GetProcessesByName(expectedProcessName);
            }

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs:644

• Process.GetProcessesByName expects the process name without an extension. Constants.MSBuildExecutableName is MSBuild.exe on Windows, so this will likely return an empty set on Windows and break CountActiveNodesWithMode (server-node self-termination / over-provisioning detection). Use an extensionless name here (e.g., Path.GetFileNameWithoutExtension(Constants.MSBuildExecutableName) or Constants.MSBuildAppName).

            var expectedProcessName = Constants.MSBuildExecutableName;

            Process[] processes;
            try
            {
                processes = Process.GetProcessesByName(expectedProcessName);
            }