fix(burn-in): add -SkipArtifact switch; restore production fail-on-missing-dist

PROBLEM: Invoke-RemoteBuild.ps1 was creating a placeholder artifact when the build
produced no dist/ directory. In production this hides build defects (build exits 0
but produces nothing = silent failure treated as success).

SOLUTION: Add explicit -SkipArtifact switch to the call chain:

Invoke-RemoteBuild.ps1:
  - Add -SkipArtifact switch. When set: skips packaging entirely (no zip created).
  - When absent (production default): missing GuestArtifactSource is a hard error
    with a clear message pointing to -SkipArtifact as the opt-in for no-output builds.

Invoke-CIJob.ps1:
  - Add -SkipArtifact switch. Passes it to Invoke-RemoteBuild.ps1.
  - Phase 6 (Get-BuildArtifacts) is entirely skipped when set; logs 'skipped' event.

Test-CapacityBurnIn.ps1:
  - Add -SkipArtifact switch. Adds '-SkipArtifact' flag to each child job argList.

Start-BurnInTest.ps1:
  - Always passes -SkipArtifact by default (burn-in uses delay commands, no output).
  - Add -NoSkipArtifact override switch for runs that do produce real artifacts.
  - Remove -NoSkipArtifact escape from wrapper: default burn-in never needs artifacts.

scripts/Invoke-RemoteBuild.ps1

@@ -126,6 +126,12 @@ param(
     # and VMware Tools HGFS driver present in the guest.
     [switch] $UseSharedCache,
 
+    # When set, skip artifact packaging and do NOT write artifacts.zip in the guest.
+    # Use for burn-in / smoke builds that intentionally produce no output.
+    # In production (default: absent) the build MUST produce GuestArtifactSource or the
+    # job fails — a silently-empty build is treated as an error.
+    [switch] $SkipArtifact,
+
     # Guest OS type — Linux uses SSH instead of WinRM.
     [ValidateSet('Windows', 'Linux')]
     [string] $GuestOS = 'Windows',
@@ -427,7 +433,7 @@ try {
         # ── Custom build command (e.g. python, msbuild, cmake…) ──────────
         Write-Host "[Invoke-RemoteBuild] Running build command: $BuildCommand"
         $buildExit = Invoke-Command -Session $session -ScriptBlock {
-            param($workDir, $cmd, $artifactZip, $artifactSrc, $useCache, $extraEnv)
+            param($workDir, $cmd, $artifactZip, $artifactSrc, $useCache, $extraEnv, $skipArt)
             # Unbuffered Python output — lines appear in real-time instead of being held in buffer
             $env:PYTHONUNBUFFERED = '1'
             if ($useCache) {
@@ -442,21 +448,14 @@ try {
             & cmd /c "$cmd 2>&1" | ForEach-Object { Write-Host $_ }
             $exit = $LASTEXITCODE
 
-            if ($exit -eq 0) {
+            if ($exit -eq 0 -and -not $skipArt) {
                 $archiveDir = Split-Path $artifactZip -Parent
                 if (-not (Test-Path $archiveDir)) {
                     New-Item -ItemType Directory -Path $archiveDir -Force | Out-Null
                 }
                 $srcPath = Join-Path $workDir $artifactSrc
                 if (-not (Test-Path $srcPath)) {
-                    # No artifact source — create a placeholder zip so that
-                    # Get-BuildArtifacts succeeds and the full pipeline is exercised.
-                    # This is expected for burn-in / smoke builds that produce no output.
-                    Write-Host "[Build] Artifact source '$srcPath' not found — creating placeholder artifact."
-                    $placeholder = Join-Path $archiveDir 'build-ok.txt'
-                    'No artifact source directory. Build command completed successfully.' |
-                        Out-File -FilePath $placeholder -Encoding UTF8
-                    Compress-Archive -Path $placeholder -DestinationPath $artifactZip -Force
+                    throw "Artifact source directory not found: $srcPath. Build succeeded (exit 0) but produced no output. Check GuestArtifactSource or use -SkipArtifact for builds that intentionally produce no files."
                 }
                 else {
                     $7zip = 'C:\Program Files\7-Zip\7z.exe'
@@ -472,7 +471,7 @@ try {
             }
 
             return $exit
-        } -ArgumentList $GuestWorkDir, $BuildCommand, $GuestArtifactZip, $GuestArtifactSource, $UseSharedCache.IsPresent, $ExtraGuestEnv
+        } -ArgumentList $GuestWorkDir, $BuildCommand, $GuestArtifactZip, $GuestArtifactSource, $UseSharedCache.IsPresent, $ExtraGuestEnv, $SkipArtifact.IsPresent
 
         if ($buildExit -ne 0) {
             throw "Build command failed (exit $buildExit)"
@@ -535,7 +534,11 @@ try {
         }
     }
 
-    Write-Host "[Invoke-RemoteBuild] Build succeeded. Artifact at $GuestArtifactZip"
+    if ($SkipArtifact) {
+        Write-Host '[Invoke-RemoteBuild] Build succeeded. Artifact packaging skipped (-SkipArtifact).'
+    } else {
+        Write-Host "[Invoke-RemoteBuild] Build succeeded. Artifact at $GuestArtifactZip"
+    }
 }
 finally {
     Remove-PSSession $session -ErrorAction SilentlyContinue