Experiment: copy test results to staging dir before publish

Copy xUnit XML test results to Build.StagingDirectory/TestResults using
CopyFiles@2 before PublishTestResults, mirroring the pattern used in
dotnet/diagnostics. This isolates the test results from the source tree
and avoids potential path or file-system differences that may be causing
PublishTestResults v2.270.0 to batch results differently than in the
diagnostics-public-ci pipeline.

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

Author: Max Charlamb

eng/pipelines/runtime-diagnostics.yml

@@ -95,11 +95,20 @@ extends:
                   unpackFolder: $(Build.SourcesDirectory)/artifacts/runtime
                   displayName: 'Runtime Build Artifacts'
             postBuildSteps:
+            # Copy test results to staging directory (preserves encoding, isolates from source tree).
+            - task: CopyFiles@2
+              displayName: 'Gather Test Results'
+              inputs:
+                sourceFolder: '$(Build.SourcesDirectory)/artifacts/TestResults'
+                contents: '**/*.xml'
+                targetFolder: '$(Build.StagingDirectory)/TestResults'
+              continueOnError: true
+              condition: always()
             - task: PublishTestResults@2
               inputs:
                 testResultsFormat: xUnit
                 testResultsFiles: '**/*.xml'
-                searchFolder: '$(Build.SourcesDirectory)/artifacts/TestResults'
+                searchFolder: '$(Build.StagingDirectory)/TestResults'
                 testRunTitle: 'Tests $(_PhaseName)'
                 failTaskOnFailedTests: true
                 publishRunAttachments: true
@@ -143,11 +152,20 @@ extends:
                   unpackFolder: $(Build.SourcesDirectory)/artifacts/runtime
                   displayName: 'Runtime Build Artifacts'
             postBuildSteps:
+            # Copy test results to staging directory (preserves encoding, isolates from source tree).
+            - task: CopyFiles@2
+              displayName: 'Gather Test Results'
+              inputs:
+                sourceFolder: '$(Build.SourcesDirectory)/artifacts/TestResults'
+                contents: '**/*.xml'
+                targetFolder: '$(Build.StagingDirectory)/TestResults'
+              continueOnError: true
+              condition: always()
             - task: PublishTestResults@2
               inputs:
                 testResultsFormat: xUnit
                 testResultsFiles: '**/*.xml'
-                searchFolder: '$(Build.SourcesDirectory)/artifacts/TestResults'
+                searchFolder: '$(Build.StagingDirectory)/TestResults'
                 testRunTitle: 'Tests $(_PhaseName)'
                 failTaskOnFailedTests: true
                 publishRunAttachments: true