Add SourceRootMappedPathsFeatureSupported property (#26095) (#26127)

src/Compilers/Core/MSBuildTask/Microsoft.Managed.Core.targets

@@ -88,7 +88,14 @@
       <SourceRoot Include="@(_MappedSourceRoot)" />
     </ItemGroup>
   </Target>
-  
+
+  <!-- 
+    Declare that target InitializeSourceRootMappedPaths that populates MappedPaths metadata on SourceRoot items is available.
+  -->
+  <PropertyGroup>
+    <SourceRootMappedPathsFeatureSupported>true</SourceRootMappedPathsFeatureSupported>
+  </PropertyGroup>
+
   <!-- 
     If InitializeSourceControlInformation target isn't supported, we just continue without invoking that synchronization target. 
     We'll proceed with SourceRoot (and other source control properties) provided by the user (or blank).

src/Compilers/Core/MSBuildTaskTests/DotNetSdkTests.cs

@@ -74,7 +74,8 @@ public void TestSourceLink()
                 {
                     "@(SourceRoot->'%(Identity): %(MappedPath)')",
                     "$(DeterministicSourcePaths)",
-                    "$(PathMap)"
+                    "$(PathMap)",
+                    "$(SourceRootMappedPathsFeatureSupported)"
                 },
                 expectedResults: new[] 
                 {
@@ -82,8 +83,9 @@ public void TestSourceLink()
                     $@"{root1}: /_/",
                     $@"{root1}sub1\: /_/sub1/",
                     $@"{root1}sub2\: /_/sub2/",
-                    @"true",
-                    $@"{root2}=/_1/,{root1}=/_/,PreviousPathMap"
+                    "true",
+                    $@"{root2}=/_1/,{root1}=/_/,PreviousPathMap",
+                    "true"
                 });
 
             AssertEx.AssertEqualToleratingWhitespaceDifferences(