fix(install): reuse probe skips the active venv; venv creation failure is fatal

Review finding: with the install's own venv activated (a re-run), `uv python
find <range>` returned venv/bin/python3, which setup_venv then deleted before
handing the dead path to `uv venv`; the stage still exited 0 and printed
"Virtual environment ready". Probe with --system so only base interpreters
qualify, and fail the stage when the venv was not created.

scripts/install.sh

@@ -692,7 +692,9 @@ check_python() {
     # No 3.11, but any interpreter inside requires-python (>=3.11,<3.14) works: reuse it rather
     # than downloading 3.11 — the download is a hard failure on hosts that cannot reach GitHub
     # releases, and the user already has a supported Python (#10778).
-    if PYTHON_PATH="$("$UV_CMD" python find "$PYTHON_SUPPORTED_RANGE" 2>/dev/null)"; then
+    # --system: with the install's own venv activated (a re-run), `uv python find` would return
+    # venv/bin/python3, which setup_venv is about to delete out from under itself.
+    if PYTHON_PATH="$("$UV_CMD" python find --system "$PYTHON_SUPPORTED_RANGE" 2>/dev/null)"; then
         PYTHON_FOUND_VERSION="$("$PYTHON_PATH" --version 2>/dev/null)"
         PYTHON_VERSION="$PYTHON_PATH"  # uv venv --python / UV_PYTHON pin onto this interpreter
         log_success "Python found: $PYTHON_FOUND_VERSION (supported; reusing instead of downloading 3.11)"
@@ -1752,8 +1754,12 @@ setup_venv() {
         rm -rf venv
     fi
 
-    # uv creates the venv and pins the Python version in one step
+    # uv creates the venv and pins the Python version in one step. Fail loudly: `set -e` does not
-    $UV_CMD venv venv --python "$PYTHON_VERSION"
+    # reach this line's callers on every path, and a missing venv used to be reported as ready.
+    if ! $UV_CMD venv venv --python "$PYTHON_VERSION" || [ ! -x "venv/bin/python" ]; then
+        log_error "Failed to create the virtual environment with Python $PYTHON_VERSION"
+        exit 1
+    fi
 
     # Neutralize any inherited UV_PYTHON (e.g. UV_PYTHON=3.14 left in the
     # user's shell env). uv honours UV_PYTHON over an existing venv for the

tests/test_install_sh_reuse_supported_python.py

@@ -45,6 +45,8 @@ def _run_prerequisites(tmp_path: Path, *, uv_find_script: str) -> subprocess.Com
 def test_supported_newer_python_is_reused_instead_of_downloading_311(tmp_path: Path) -> None:
     """3.11 absent, 3.13 present: the installer must take 3.13 and never call ``uv python install``."""
     result = _run_prerequisites(tmp_path, uv_find_script=(
-        f"  [ \"$3\" = 3.11 ] && exit 2\n  [ \"$3\" = '>=3.11,<3.14' ] && {{ echo {tmp_path}/bin/python3.13; exit 0; }}\n  exit 2\n"))
+        # The range probe must carry --system: with the install's own venv activated, a plain
+        # `uv python find` returns venv/bin/python3, which setup_venv then deletes.
+        f"  [ \"$3\" = 3.11 ] && exit 2\n  [ \"$3\" = --system ] && [ \"$4\" = '>=3.11,<3.14' ] && {{ echo {tmp_path}/bin/python3.13; exit 0; }}\n  exit 2\n"))
     assert "DOWNLOAD ATTEMPTED" not in result.stdout, result.stdout
     assert "Python found: Python 3.13.12" in result.stdout, result.stdout