Fix package validation telemetry assembly resolution warnings

[JanProvaznik]

Summary

During build, package validation emits warnings about being unable to resolve Microsoft.VisualStudio.Telemetry.dll:

Could not resolve reference 'Microsoft.VisualStudio.Telemetry.dll' directly or transitively referenced by 'ref/net472/Microsoft.Build.dll' (...) in any of the provided search directories.

This occurs because Microsoft.Build.Framework references Microsoft.VisualStudio.Telemetry with PrivateAssets="all" for net472 (to enable telemetry in Visual Studio scenarios). When package validation runs on packages like Microsoft.Build that depend on Framework, it cannot resolve this private reference.

Fix

Add the telemetry package path to ApiCompatAdditionalSearchPaths in Directory.Build.targets, allowing the package validation to locate the assembly.

Testing

• Built locally with .\build.cmd -v quiet - warnings no longer appear
• Build succeeds with 0 errors

[Copilot]

Pull request overview

Adds an additional API compat search path so package validation can resolve Microsoft.VisualStudio.Telemetry.dll (referenced privately by Microsoft.Build.Framework on net472) and stop emitting resolution warnings during builds.

Changes:

• Extends ApiCompatAdditionalSearchPaths when package validation is enabled to include the Telemetry package’s net472 assembly directory.
• Documents the rationale in Directory.Build.targets to clarify why the extra search path is needed.

[ViktorHofer]

As the previous owner of APICompat & PackageValidation, a quick comment. Note that this message will get emitted with low importance starting with 11.0 as these additional assemblies aren't essential for package validation to perform correctly and in most cases just add noise.