Skip PNSE doc-source P2P when local targeting pack is unavailable

When UseCompilerGeneratedDocXmlFile is true (the default) and a library
is a PNSE assembly, intellisense.targets adds a self-referencing
ProjectReference with SetTargetFramework=net11.0 to pull enriched XML
docs from the non-PNSE sibling build. In the NET481 build leg the local
targeting pack (FrameworkList.xml) is never produced, so the inner
net11.0 build fails with:

  The shared framework must be built before the local targeting pack
  can be consumed.

Guard the AddProjectReferenceToPNSEDocSource target with an Exists check
on the same FrameworkList.xml sentinel that targetingpacks.targets
already uses. When the file is absent the target is skipped and the PNSE
assembly builds without doc enrichment, which is the expected behavior
for partial-build configurations like the NET481 leg.

Fixes #127007

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: danmoseley

eng/intellisense.targets

@@ -122,11 +122,14 @@
     </PropertyGroup>
   </Target>
 
+  <!-- Skip the PNSE doc-source self-reference when the local NetCoreApp ref pack hasn't been built
+       (e.g. in NET481-only legs); doc enrichment from the sibling TFM is optional in that case. -->
   <Target Name="AddProjectReferenceToPNSEDocSource"
           DependsOnTargets="GetPNSEDocTargetFramework"
           BeforeTargets="PrepareForBuild"
           Condition="'$(IsPlatformNotSupportedAssembly)' == 'true' and
-                    '$(UseCompilerGeneratedDocXmlFile)' == 'true'">
+                    '$(UseCompilerGeneratedDocXmlFile)' == 'true' and
+                    Exists('$(MicrosoftNetCoreAppRefPackDataDir)FrameworkList.xml')">
 
     <Error Condition="'$(PNSEDocTargetFramework)' == ''"
            Text="Could not find a compatible TargetFramework to use as the documentation source for this PNSE build. Set 'PNSEDocTargetFramework' to the desired target framework." />