.NET: Foundry.Hosting IT: avoid MSB3026 in publish; fix telemetry UT flake (#5689)

CI publish step: gate the BuildProjectReferences=false fast-path on an explicit -UsePrebuiltProjectReferences switch (passed by the workflow) instead of marker detection. Adds a preflight error when stale obj/Release/net10.0 outputs would cause CS0579, with actionable recovery instructions.

Telemetry UT flake: AgentFrameworkResponseHandlerTelemetryTests was using a plain List<Activity> for OTel's InMemoryExporter. The exporter writes from background Activity completion callbacks while parallel tests on the same global ActivitySource feed every listener, racing against the assertion's enumeration and throwing 'Collection was modified'. Replaced with a small thread-safe ConcurrentActivityList that locks add/enumerate and returns a snapshot for assertions.

.github/workflows/dotnet-build-and-test.yml

@@ -379,6 +379,14 @@ jobs:
       # We rebuild and push the test container image on every IT run so framework code changes
       # are picked up; the image tag is content-hashed across the test container source AND its
       # framework project references, so identical content is a no-op push.
+      #
+      # `-UsePrebuiltProjectReferences` opts into the no-rebuild fast path: publish skips
+      # rebuilding ProjectReferences and consumes the DLLs the prior "Build Foundry hosted IT
+      # (and its deps)" step already produced. This avoids MSB3026 ("file is being used by
+      # another process") collisions caused by the previous build's shared-compilation server
+      # still holding file handles to those DLLs. Safe in CI because the prebuild step ran in
+      # the same job against the same source. Do not remove the prebuild step (the subsequent
+      # `dotnet test --no-build` step depends on it too).
       - name: Build and push Foundry Hosted Agents test container
         id: build-foundry-hosted-image
         shell: pwsh
@@ -388,7 +396,7 @@ jobs:
           if ([string]::IsNullOrWhiteSpace($registry)) {
             throw "IT_HOSTED_AGENT_REGISTRY not set in the integration environment."
           }
-          & "${{ github.workspace }}/dotnet/tests/Foundry.Hosting.IntegrationTests/scripts/it-build-image.ps1" -Registry $registry | Tee-Object -FilePath $env:GITHUB_ENV -Append
+          & "${{ github.workspace }}/dotnet/tests/Foundry.Hosting.IntegrationTests/scripts/it-build-image.ps1" -Registry $registry -UsePrebuiltProjectReferences | Tee-Object -FilePath $env:GITHUB_ENV -Append
 
       - name: Run Foundry Hosted Agents Integration Tests
         shell: pwsh