Move ErrorUtilities_Tests to Assumed_Tests

[DustinCampbell]

ErrorUtilities_Tests.cs tested Assumed.True APIs, not ErrorUtilities. Consolidate these tests into Assumed_Tests.cs in Framework.UnitTests, updating from xUnit Assert to Shouldly and adding coverage for:

• True(bool, string?) with a non-null message (both passing and failing)
• Lazy evaluation of the TrueInterpolatedStringHandler: verifies the interpolated string is not formatted when the condition is true, is formatted when false, and formats multiple args correctly

Remove ErrorUtilities_Tests.cs and its links from Build.UnitTests, Tasks.UnitTests, and Utilities.UnitTests.

[Copilot]

Pull request overview

This PR consolidates tests that were validating Assumed.True behavior (including interpolated string handler laziness) by moving them out of ErrorUtilities_Tests.cs into Framework.UnitTests’s Assumed_Tests.cs, and removes the now-misleading shared test file and its compile links from other test projects.

Changes:

• Move/merge Assumed.True-focused tests into src/Framework.UnitTests/Assumed_Tests.cs, updating assertions to Shouldly and expanding interpolated-string-handler coverage.
• Delete src/Shared/UnitTests/ErrorUtilities_Tests.cs (which wasn’t testing ErrorUtilities) and remove <Compile Include=...> links from multiple unit test projects.
• Keep the shared unit test projects clean by avoiding redundant/incorrectly-scoped shared test files.

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/Framework.UnitTests/Assumed_Tests.cs	Adds/hosts the migrated Assumed.True tests and new interpolated-string-handler laziness scenarios.
src/Shared/UnitTests/ErrorUtilities_Tests.cs	Removes the miscategorized shared test file.
src/Build.UnitTests/Microsoft.Build.Engine.UnitTests.csproj	Removes the shared compile include for the deleted test file.
src/Tasks.UnitTests/Microsoft.Build.Tasks.UnitTests.csproj	Removes the shared compile include for the deleted test file.
src/Utilities.UnitTests/Microsoft.Build.Utilities.UnitTests.csproj	Removes the shared compile include for the deleted test file.

Comments suppressed due to low confidence (1)

src/Framework.UnitTests/Assumed_Tests.cs:106

• PR description says to add coverage for Assumed.True(bool, string?) with a non-null message in both passing and failing cases. The passing case was added, but there's no test verifying the custom message is included in the thrown InternalErrorException when the condition is false (this coverage was previously in ErrorUtilities_Tests).

    [Fact]
    public void True_DoesNotThrow_WithMessage_WhenTrue()
    {
        Assumed.True(true, "msbuild rules");
    }

    [Fact]
    public void True_Throws_WhenFalse()
    {
        Should.Throw<InternalErrorException>(() => Assumed.True(false));
    }