fix(whisper-medusa): pin available portable Python

The standalone Python release used by libbackend does not publish a 3.11.18 artifact. Pin Whisper-Medusa to the available 3.11.13 build and cover the generated download URL.

Assisted-by: Codex:gpt-5 [systematic-debugging]

backend/python/whisper-medusa/install.sh

@@ -9,4 +9,5 @@ else
 fi
 
 PYTHON_VERSION="3.11"
+PYTHON_PATCH="13"
 installRequirements

backend/python/whisper-medusa/test.sh

@@ -1,3 +1,26 @@
 #!/bin/bash
 set -e
 python3 -m unittest test_unit.py
+
+tmp_dir=$(mktemp -d)
+trap 'rm -rf "$tmp_dir"' EXIT
+mkdir -p "$tmp_dir/bin"
+cat >"$tmp_dir/bin/curl" <<'EOF'
+#!/bin/bash
+printf '%s\n' "${@: -1}" >"$PORTABLE_PY_URL_CAPTURE"
+exit 22
+EOF
+chmod +x "$tmp_dir/bin/curl"
+
+PORTABLE_PY_URL_CAPTURE="$tmp_dir/url" \
+    ENV_DIR="$tmp_dir/backend" \
+    PORTABLE_PYTHON=true \
+    PATH="$tmp_dir/bin:$PATH" \
+    bash install.sh >/dev/null 2>&1 || true
+
+expected_url="https://github.com/astral-sh/python-build-standalone/releases/download/20250818/cpython-3.11.13+20250818-x86_64-unknown-linux-gnu-install_only.tar.gz"
+actual_url=$(cat "$tmp_dir/url")
+if [ "$actual_url" != "$expected_url" ]; then
+    echo "unexpected portable Python URL: $actual_url" >&2
+    exit 1
+fi