Fix nvidia-targeted-stress failing with DCGM_ST_IN_USE (-34)

nvvs (DCGM validation suite) survives when dcgmi is killed mid-run,
leaving the GPU occupied. The next dcgmi diag invocation then fails
with "affected resource is in use".

Two-part fix:
- Add nvvs and dcgmi to KillTestWorkers patterns so they are cleaned
  up by the global cancel handler
- Call KillTestWorkers at the start of RunNvidiaTargetedStressValidatePack
  to clear any stale processes before dcgmi diag runs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/platform/kill_workers.go

@@ -15,6 +15,10 @@ var workerPatterns = []string{
 	"stress-ng",
 	"stressapptest",
 	"memtester",
+	// DCGM diagnostic workers — nvvs is spawned by dcgmi diag and survives
+	// if dcgmi is killed mid-run, leaving the GPU occupied (DCGM_ST_IN_USE).
+	"nvvs",
+	"dcgmi",
 }
 
 // KilledProcess describes a process that was sent SIGKILL.