Consolidate duplicate MSBuildUtilities copies (#51487)

[mthalman]

Why

MSBuildUtilities existed as two copies whose class bodies were byte-for-byte identical, differing only by namespace:

• src/Common/MSBuildUtilities.cs in Microsoft.DotNet.Cli, compiled into the SDK MSBuild build tasks.
• src/Cli/Microsoft.DotNet.FileBasedPrograms/MSBuildUtilities.cs in Microsoft.DotNet.FileBasedPrograms, shipped in the source package and consumed by Roslyn, the dotnet CLI, and ProjectTools.

Keeping two copies in sync is exactly the duplication tracked by #51487.

What

Collapse the two copies into the single physical file src/Common/MSBuildUtilities.cs and switch the namespace at compile time with an MSBUILD_BUILD_TASKS symbol:

• The default form (no symbol) is the one that originally shipped in the source package: namespace Microsoft.DotNet.FileBasedPrograms, with #nullable enable and using System;. External and uncontrolled consumers (Roslyn, the dotnet CLI) compile the shipped contentFiles with no symbols defined, so this default keeps their input unchanged.
• The two build-tasks projects (Microsoft.NET.Build.Tasks and Microsoft.NET.Build.Extensions.Tasks) define MSBUILD_BUILD_TASKS, which selects the Microsoft.DotNet.Cli namespace and drops the nullable/using directives. Those projects enable nullable and provide System via ImplicitUsings, so emitting the directives there would otherwise trip IDE0240/IDE0005.

The class body is unchanged, so the emitted IL for ConvertStringToBool is identical to before. Net result is +27 / -72 with one shared file instead of two.

Validation

• Built every consumer clean (0/0): both build-tasks projects (net11.0 + net472), Microsoft.DotNet.ProjectTools, the Microsoft.DotNet.FileBasedPrograms source package, and the dotnet CLI.
• Inspected the compiled assemblies' metadata to confirm per-assembly namespaces: the two build-tasks DLLs expose Microsoft.DotNet.Cli.MSBuildUtilities; Microsoft.DotNet.ProjectTools.dll exposes Microsoft.DotNet.FileBasedPrograms.MSBuildUtilities.
• Confirmed the shipped source package still packs MSBuildUtilities.cs into contentFiles/cs/{net9.0,netstandard2.0}/ defaulting to the Microsoft.DotNet.FileBasedPrograms namespace, matching the prior shipped contract.
• Ran the build-tasks unit tests that exercise ConvertStringToBool (ProcessFrameworkReferences: 39/39 passing).

InternalAPI.Unshipped.txt is unchanged because the default namespace remains Microsoft.DotNet.FileBasedPrograms.

Contributes to #51487

[Copilot]

Pull request overview

This PR removes the duplicated MSBuildUtilities source by consolidating both byte-identical copies into src/Common/MSBuildUtilities.cs, using an MSBUILD_BUILD_TASKS compilation symbol to select the appropriate namespace for each consumer (build tasks vs. FileBasedPrograms source package).

Changes:

• Added MSBUILD_BUILD_TASKS constant to the two MSBuild build-task projects so the shared file compiles into Microsoft.DotNet.Cli.
• Updated src/Common/MSBuildUtilities.cs to switch namespaces (and file-level directives) via #if MSBUILD_BUILD_TASKS.
• Removed the old FileBasedPrograms-local MSBuildUtilities.cs and linked the shared src/Common/MSBuildUtilities.cs into the FileBasedPrograms shared items.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Tasks/Microsoft.NET.Build.Tasks/Microsoft.NET.Build.Tasks.csproj	Defines MSBUILD_BUILD_TASKS so the shared utility compiles under the build-task namespace.
src/Tasks/Microsoft.NET.Build.Extensions.Tasks/Microsoft.NET.Build.Extensions.Tasks.csproj	Adds MSBUILD_BUILD_TASKS alongside existing constants for consistent build-task compilation.
src/Common/MSBuildUtilities.cs	Centralizes the implementation and conditionally selects namespace/directives per consumer.
src/Cli/Microsoft.DotNet.FileBasedPrograms/MSBuildUtilities.cs	Deletes the redundant local copy now that the file is shared.
src/Cli/Microsoft.DotNet.FileBasedPrograms/Microsoft.DotNet.FileBasedPrograms.projitems	Links the shared src/Common/MSBuildUtilities.cs into the FileBasedPrograms source package inputs.