.NET: Foundry.Hosted IT - fix MSBuild parallel-output races

Two surgical changes inside the dotnet-foundry-hosted-it job:

1. Replace dotnet build <slnx> -f net10.0 with dotnet build <test.csproj>. The test csproj pins TargetFrameworks=net10.0 and its ProjectReference closure gives MSBuild a single-rooted graph, eliminating the duplicate inner-builds that race on bin/obj. Drops the two New-FilteredSolution.ps1 steps.

2. In it-build-image.ps1, drop the -UsePrebuiltProjectReferences switch and always pass --no-dependencies to dotnet publish. Publish now resolves TestContainer's framework refs by reading prebuilt DLLs and never re-touches them. Replaces the partial-mitigation in PR #5689 with a structural fix.

Local validation confirmed published Foundry.dll has identical mtime and bytes as the prebuild output.

dotnet/tests/Foundry.Hosting.IntegrationTests/scripts/it-build-image.ps1

@@ -41,14 +41,7 @@ param(
 
     [string] $Repository = "foundry-hosting-it",
 
-    [string] $TestContainerProject = "dotnet/tests/Foundry.Hosting.IntegrationTests.TestContainer",
-
-    # Explicit opt-in for the no-rebuild fast path. CI sets this after running the
-    # "Build Foundry hosted IT (and its deps)" step, which guarantees the prebuilt
-    # library DLLs match current source. Off by default so local invocations always
-    # let publish rebuild ProjectReferences and never produce an image whose tag is
-    # computed from current source while the contents come from a stale build.
-    [switch] $UsePrebuiltProjectReferences
+    [string] $TestContainerProject = "dotnet/tests/Foundry.Hosting.IntegrationTests.TestContainer"
 )
 
 $ErrorActionPreference = "Stop"
@@ -107,60 +100,35 @@ if (Test-Path $out) {
     Remove-Item -Recurse -Force $out
 }
 
-# Conditionally tell publish to skip rebuilding ProjectReferences and consume the
-# prebuilt library DLLs in place. This avoids two failure modes that arise when
-# the CI workflow runs a `dotnet build` of the same library projects immediately
-# before this script:
-#   1) MSB3026 "file is being used by another process" when publish's MSBuild
-#      tries to overwrite src/<lib>/bin/Release/net10.0/<lib>.dll while the
-#      previous build's shared-compilation server still holds a file handle.
-#   2) Publish needlessly rebuilding identical managed (RID-agnostic) library
-#      DLLs that prebuild already produced.
-# Gated on -UsePrebuiltProjectReferences (a strict opt-in) instead of marker
-# detection, because a developer machine may have a stale Release build of the
-# libraries from days ago; using those would silently produce an image whose
-# content is older than the source the tag is computed from.
-$publishExtraArgs = @()
-if ($UsePrebuiltProjectReferences) {
-    Write-Host "-UsePrebuiltProjectReferences: skipping ProjectReference rebuild." -ForegroundColor DarkGray
-    $publishExtraArgs += "-p:BuildProjectReferences=false"
-} else {
-    # Preflight: in default (rebuild) mode, publish propagates RuntimeIdentifier=linux-musl-x64
-    # to library ProjectReferences and writes their intermediates to a RID-suffixed obj path
-    # (e.g. obj/Release/net10.0/linux-musl-x64/). DefaultItemExcludes follows the new
-    # IntermediateOutputPath, so any *.AssemblyInfo.cs left in obj/Release/net10.0/ from a
-    # prior `dotnet build` is no longer excluded and gets picked up by the **/*.cs Compile
-    # glob, producing CS0579 "duplicate attribute" errors. Detect that state up front and
-    # tell the user exactly how to recover.
-    $staleObjProbes = @(
-        "dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/obj/Release/net10.0",
-        "dotnet/src/Microsoft.Agents.AI.Foundry/obj/Release/net10.0",
-        "dotnet/src/Microsoft.Agents.AI/obj/Release/net10.0",
-        "dotnet/src/Microsoft.Agents.AI.Abstractions/obj/Release/net10.0"
-    )
-    $stale = @($staleObjProbes | Where-Object { Test-Path (Join-Path $_ "*.AssemblyInfo.cs") })
-    if ($stale.Count -gt 0) {
-        $msg = @(
-            "Detected prior Release/net10.0 build outputs in:"
-            ($stale | ForEach-Object { "  - $_" })
-            ""
-            "Publish would propagate -r linux-musl-x64 to those ProjectReferences and the"
-            "leftover obj/Release/net10.0/*.AssemblyInfo.cs files would cause CS0579 duplicate"
-            "attribute errors. Pick one:"
-            "  (a) Pass -UsePrebuiltProjectReferences (skips ProjectReference rebuild and"
-            "      uses the existing src/<lib>/bin/Release/net10.0/*.dll outputs in place)."
-            "      Only safe when you know those DLLs match current source - this is the path"
-            "      CI uses immediately after its 'Build Foundry hosted IT (and its deps)' step."
-            "  (b) Remove the stale obj/Release trees, e.g.:"
-            "        Remove-Item -Recurse -Force dotnet/src/Microsoft.Agents.AI*/obj/Release"
-            "      and re-run."
-        ) -join "`n"
-        throw $msg
-    }
-    Write-Host "Letting publish build ProjectReferences (pass -UsePrebuiltProjectReferences in CI to skip)." -ForegroundColor DarkGray
+# Always tell publish to skip ProjectReference rebuilds via --no-dependencies. Publish
+# resolves TestContainer's framework lib references (Foundry, Foundry.Hosting and their
+# transitive deps) by reading the prebuilt DLLs at src/<lib>/bin/Release/net10.0/*.dll.
+# This:
+#   1) Structurally avoids the MSB3026 "file is being used by another process" race that
+#      occurs when publish overwrites the same DLL paths a prior `dotnet build` produced
+#      while VBCSCompiler from that build still holds file handles.
+#   2) Avoids needlessly rebuilding identical managed (RID-agnostic) library DLLs.
+# Callers MUST run `dotnet build dotnet/tests/Foundry.Hosting.IntegrationTests/Foundry.Hosting.IntegrationTests.csproj -c Release`
+# (or equivalent) first so those prebuilt DLLs exist. The CI workflow does this in the
+# preceding "Build Foundry hosted IT (and its deps)" step.
+$prebuildProbes = @(
+    "dotnet/src/Microsoft.Agents.AI.Foundry/bin/Release/net10.0/Microsoft.Agents.AI.Foundry.dll",
+    "dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/bin/Release/net10.0/Microsoft.Agents.AI.Foundry.Hosting.dll"
+)
+$missingPrebuilds = @($prebuildProbes | Where-Object { -not (Test-Path $_) })
+if ($missingPrebuilds.Count -gt 0) {
+    $msg = @(
+        "Required prebuilt outputs not found:"
+        ($missingPrebuilds | ForEach-Object { "  - $_" })
+        ""
+        "Publish runs with --no-dependencies and consumes prebuilt DLLs in place. Build the"
+        "test project first so its ProjectReference closure populates src/<lib>/bin/Release/net10.0/:"
+        "  dotnet build dotnet/tests/Foundry.Hosting.IntegrationTests/Foundry.Hosting.IntegrationTests.csproj -c Release"
+    ) -join "`n"
+    throw $msg
 }
 
-dotnet publish $TestContainerProject -c Release -f net10.0 -r linux-musl-x64 --self-contained false -o $out @publishExtraArgs --tl:off | Out-Host
+dotnet publish $TestContainerProject -c Release -f net10.0 -r linux-musl-x64 --self-contained false --no-dependencies -o $out --tl:off | Out-Host
 if ($LASTEXITCODE -ne 0) {
     throw "dotnet publish failed with exit code $LASTEXITCODE."
 }