Change PrivateAssets from "all" to "runtime" in MSBL001 diagnostic

[Copilot]

Requiring PrivateAssets="all" can break downstream projects that need compile-time references to MSBuild packages. Only PrivateAssets="runtime" is actually needed since ExcludeAssets="runtime" already handles the runtime exclusion.

Changes

• src/MSBuildLocator/build/Microsoft.Build.Locator.targets - Updated MSBL001 error message to prescribe PrivateAssets="runtime"
• docs/diagnostics/MSBL001.md - Updated documentation and examples to use PrivateAssets="runtime"

Recommended PackageReference

<PackageReference Include="Microsoft.Build" ExcludeAssets="runtime" PrivateAssets="runtime" />

Original prompt

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This section details on the original issue you should resolve

<issue_title>MSBuild locator warning should not prescribe PrivateAssets="all"</issue_title>
<issue_description>The new check for ExcludeAssets="compile" is fine, but the error message shouldn't tell the user to set PrivateAssets="all" when in fact only PrivateAssets="runtime" is required, since that can introduce build breaks in referencing packages/projects due to a missing reference assembly to msbuild packages.

MSBuildLocator/src/MSBuildLocator/build/Microsoft.Build.Locator.targets

Line 22 in 44b8f1e

Text="A PackageReference to the package '%(_DistinctMSBuildPackagesReferencedPoorly.NuGetPackageId)' at version '%(_DistinctMSBuildPackagesReferencedPoorly.NuGetPackageVersion)' is present in this project without ExcludeAssets="runtime" and PrivateAssets="all" set. This can cause errors at run-time due to MSBuild assembly-loading."

</issue_description>

<agent_instructions>Update the detection, the emitted diagnostic message, and the help documentation to move to using PrivateAssets="runtime" instead of PrivateAssets="all"</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes MSBuild locator warning should not prescribe PrivateAssets="all" #368

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