fix: replace asyncio subprocess proxy with thread-based Popen proxy

asyncio subprocess PIPE unreliable for long-lived stdio bridging. Use Popen + threads instead. Also fix smoke_test.sh stdin handling.

Dockerfile

@@ -27,7 +27,7 @@ RUN pip install --upgrade pip build \
 # ── Stage 3: final runtime ────────────────────────────────────────────────────
 # Use Microsoft's official .NET runtime image — avoids the SHA1-signed APT key
 # issue on newer Debian hosts (trixie+ rejects packages.microsoft.com GPG since 2026-02-01).
-FROM mcr.microsoft.com/dotnet/runtime:8.0
+FROM mcr.microsoft.com/dotnet/runtime:10.0
 
 # Install Python 3 + pip (the dotnet base image is Debian bookworm)
 RUN apt-get update \