ci: dump managed thread stacks when the nethtest watchdog fires

Flat stateTest chunks intermittently stall at startup with zero
progress; capture the deadlocked stacks before terminating.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Marchhill

.github/workflows/run-nethtest.yml

@@ -625,6 +625,12 @@ jobs:
             echo ""
             echo "--- recent nethtest status ---"
             tail -30 "$STDERR_FILE" | sed 's/\x1b\[[0-9;]*m//g' || true
+            echo ""
+            echo "--- managed thread stacks (deadlock diagnostics) ---"
+            if kill -0 "$NETHTEST_PID" 2>/dev/null; then
+              dotnet tool install --global dotnet-stack >/dev/null 2>&1 || true
+              "$HOME/.dotnet/tools/dotnet-stack" report -p "$NETHTEST_PID" 2>&1 | head -600 || true
+            fi
             echo "================================"
           }