docs: document python3-venv prereq on the coordinator host

The preflight job creates a venv to install the labgrid-client fork;
modern Debian/Ubuntu ships `python3-venv` as a separate package and
omits it from default installs, so the first run on a fresh
coordinator host fails with "ensurepip is not available" until the
admin runs `apt install python3-venv`.  Surface this in the runner
prerequisites section and make the probe's error message name the
exact fix.

Author: tfcollins

.github/workflows/hardware-test.yml

@@ -51,7 +51,11 @@ jobs:
           VENV="$HOME/.cache/adidt-ci/labgrid-venv"
           if [[ ! -x "$VENV/bin/labgrid-client" ]]; then
             echo "Creating labgrid venv at $VENV" >&2
-            python3 -m venv "$VENV"
+            if ! python3 -m venv "$VENV" 2>&1; then
+              echo "venv creation failed — python3-venv package likely missing." >&2
+              echo "Install on the coordinator host:  sudo apt-get install -y python3-venv python3-full" >&2
+              exit 1
+            fi
             "$VENV/bin/pip" install --quiet --disable-pip-version-check --upgrade pip
             "$VENV/bin/pip" install --quiet --disable-pip-version-check "$LABGRID_PIP"
           fi

doc/source/developer/hardware_ci.rst

@@ -162,6 +162,15 @@ System-tool prerequisites on each hw-node runner:
    # For ZCU102/AD9081 (USB SD-mux mode):
    sudo apt-get install -y usbsdmux
 
+On the **coordinator host** (the one labeled ``hw-coordinator``), the
+preflight job creates a venv to host the labgrid-client install.
+Modern Debian/Ubuntu ships ``python3-venv`` separately, so install
+the matching package before the first run:
+
+.. code-block:: bash
+
+   sudo apt-get install -y python3-venv python3-full
+
 Coordinator-mode tests additionally require SSH key-auth from the
 ``hw-coordinator`` runner to the exporter host (for the
 ``MassStorageDriver`` SSH-proxy path) and write access to