fix(burn-in): create placeholder artifact when build command produces no output dir

Invoke-RemoteBuild.ps1: when BuildCommand succeeds (exit 0) but GuestArtifactSource
(default: dist) does not exist, instead of silently skipping packaging, now creates a
placeholder 'build-ok.txt' in the artifact output dir and zips it. This ensures
Get-BuildArtifacts.ps1 always finds artifacts.zip regardless of whether the build
produced real output files. Correct for burn-in, smoke, and delay-only build commands.

Start-BurnInTest.ps1: revert default BuildCommand to 'ping 127.0.0.1 -n 31 > nul'
(~30s delay, no artifact output) now that the pipeline handles missing dist gracefully.
The previous 'if not exist dist mkdir dist' workaround is no longer needed.

scripts/Start-BurnInTest.ps1

@@ -34,10 +34,12 @@
 
 .PARAMETER BuildCommand
     Comando da eseguire nel guest come passo build (eseguito via cmd /c nel guest).
-    Default: crea la directory dist con un file marker, poi esce 0 — esercita
-    l'intero pipeline inclusa la fase di packaging e artifact collection.
-    Usare 'ping 127.0.0.1 -n 121 > nul' per mantenere 4 VM attive ~120s
-    e osservare la concorrenza senza produrre artifact.
+    Default: 'ping 127.0.0.1 -n 31 > nul' (attesa ~30s, nessun output reale).
+    Quando il build command non produce la directory dist, Invoke-RemoteBuild crea
+    automaticamente un artifact placeholder (build-ok.txt in zip) per permettere
+    all'intero pipeline di completarsi.
+    Usare 'ping 127.0.0.1 -n 601 > nul' per mantenere 4 VM attive ~600s
+    e osservare la concorrenza prolungata.
 
 .PARAMETER RoundTimeoutMinutes
     Minuti massimi di attesa per ogni round prima di interrompere i job rimasti.
@@ -46,7 +48,7 @@
 param(
     [int]    $Rounds              = 3,
     [int]    $Parallelism         = 4,
-    [string] $BuildCommand        = 'if not exist dist mkdir dist & echo burnin_ok > dist\marker.txt',
+    [string] $BuildCommand        = 'ping 127.0.0.1 -n 31 > nul',
     [int]    $RoundTimeoutMinutes = 60
 )