Make DOTNET_CLI_USE_MSBUILD_SERVER=false authoritative opt-out

[Copilot]

DOTNET_CLI_USE_MSBUILD_SERVER=false previously left MSBUILDUSESERVER unset, allowing /mt, response files, or MSBUILDFORCEMULTITHREADED=1 to implicitly enable the MSBuild server.

• Server selection

  • Forward MSBUILDUSESERVER=0 when the CLI server setting is disabled.
  • Preserve explicitly configured MSBUILDUSESERVER values.

• Coverage

  • Cover unset, explicitly enabled, and explicitly disabled MSBUILDUSESERVER cases.

DOTNET_CLI_USE_MSBUILD_SERVER=false
MSBUILDUSESERVER=<unset>
→ MSBUILDUSESERVER=0

• Fixes DOTNET_CLI_USE_MSBUILD_SERVER=false is not an authoritative opt-out for /mt #55392

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[Copilot]

Pull request overview

This PR makes DOTNET_CLI_USE_MSBUILD_SERVER=false an authoritative opt-out by ensuring the CLI forwards an explicit MSBUILDUSESERVER=0 when the user hasn’t explicitly set MSBUILDUSESERVER, preventing implicit server enablement via /mt, response files, or MSBUILDFORCEMULTITHREADED.

Changes:

• Change MSBuild forwarding to set MSBUILDUSESERVER to 1 or 0 based on DOTNET_CLI_USE_MSBUILD_SERVER when MSBUILDUSESERVER is not already set.
• Evaluate DOTNET_CLI_USE_MSBUILD_SERVER per forwarding-app instance (not once at type initialization).
• Add unit tests covering the interaction between DOTNET_CLI_USE_MSBUILD_SERVER and explicit MSBUILDUSESERVER values.

Reviewed changes

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

File	Description
src/Cli/Microsoft.DotNet.Cli.Utils/MSBuildForwardingAppWithoutLogging.cs	Sets MSBUILDUSESERVER based on DOTNET_CLI_USE_MSBUILD_SERVER unless user explicitly sets MSBUILDUSESERVER; evaluates setting per invocation.
test/dotnet.Tests/CommandTests/MSBuild/GivenMsbuildForwardingApp.cs	Adds tests validating forwarding behavior for unset/explicit MSBUILDUSESERVER when CLI setting disables the server.