fix(scripts): probe _pick_python candidates through env so make dev starts the frontend on Windows (#5181)

* bugfix #5179

* test: cover the env-aware _pick_python fallback from #5179

Follow the test_serve_nginx_stop.py extraction pattern: drive the real
_pick_python from serve.sh against a stub-only PATH plus a mocked env.

- python3 succeeds directly but fails through env -> python selected
  (red on main, green on this branch)
- env rejects every candidate -> nonzero exit (also red on main)
- healthy PATH with the real env -> python3 preferred, guarding against
  over-rejection

MSYS/Git Bash hosts need the stub dir as an MSYS-style (/c/...) PATH
entry, and bash diagnostics may arrive in the console code page, so the
runner decodes output with errors="replace".

backend/tests/test_serve_pick_python.py

@@ -0,0 +1,120 @@
+"""Regression coverage for #5179: on Windows/Git Bash the Microsoft Store
+python alias stubs pass Bash's own PATH lookup but cannot be exec'd through
+/usr/bin/env, so `make dev` never started the frontend."""
+
+from __future__ import annotations
+
+import shlex
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+SERVE_SH = REPO_ROOT / "scripts" / "serve.sh"
+
+
+def _extract_shell_function(name: str) -> str:
+    text = SERVE_SH.read_text(encoding="utf-8")
+    marker = f"{name}() {{"
+    start = text.index(marker)
+    depth = 0
+    chunks: list[str] = []
+
+    for line in text[start:].splitlines(keepends=True):
+        chunks.append(line)
+        depth += line.count("{") - line.count("}")
+        if depth == 0:
+            return "".join(chunks)
+
+    raise AssertionError(f"Could not extract shell function {name}")
+
+
+# Shells out to bash with a stub-only PATH plus an optional `env` override,
+# exactly the split that broke Windows: the candidate stubs succeed when
+# invoked directly, while the mocked `env` mimics /usr/bin/env resolving the
+# WindowsApps alias (exit 127 = cannot exec).
+_SCRIPT_TEMPLATE = r"""
+set -u
+BIN=__BIN__
+mkdir -p "$BIN"
+for name in __STUBS__; do
+    printf '#!/bin/sh
+exit 0
+' > "$BIN/$name"
+    chmod +x "$BIN/$name"
+done
+export PATH="$BIN:$PATH"
+
+__ENV_MOCK__
+
+__FUNCTION__
+
+_pick_python
+"""
+
+
+def _to_bash_path(path: Path) -> str:
+    """Return `path` in the form the target bash understands.
+
+    MSYS/Git Bash needs an MSYS-style path (/c/...) on PATH: a drive-letter
+    style segment (C:/...) is resolved inconsistently between bash's own
+    lookup and /usr/bin/env. POSIX hosts pass through unchanged.
+    """
+    posix = path.resolve().as_posix()
+    if len(posix) > 1 and posix[1] == ":":
+        return "/" + posix[0].lower() + posix[2:]
+    return posix
+
+
+def _run_pick_python(tmp_path: Path, *, env_mock: str = "") -> subprocess.CompletedProcess:
+    bash = shutil.which("bash")
+    if bash is None:
+        pytest.skip("bash is required to exercise serve.sh helpers")
+
+    script = _SCRIPT_TEMPLATE.replace("__BIN__", shlex.quote(_to_bash_path(tmp_path / "bin"))).replace("__STUBS__", "python3 python py").replace("__ENV_MOCK__", env_mock).replace("__FUNCTION__", _extract_shell_function("_pick_python"))
+    # errors="replace": bash's diagnostics may arrive in the console's code
+    # page (GBK on a Chinese Windows host) while the selection output is ASCII.
+    return subprocess.run(
+        [bash, "-c", script],
+        capture_output=True,
+        encoding="utf-8",
+        errors="replace",
+        check=False,
+    )
+
+
+def test_pick_python_prefers_python3_when_env_agrees(tmp_path):
+    # Real env: all stubs are ordinary executables, so python3 wins.
+    result = _run_pick_python(tmp_path)
+
+    assert result.returncode == 0
+    assert result.stdout.strip() == "python3"
+
+
+def test_pick_python_skips_candidate_that_env_cannot_exec(tmp_path):
+    # Store-alias world: python3 execs fine directly but env fails on it;
+    # python works through both paths. Must select python, not python3.
+    env_mock = """
+env() {
+    case "$1" in
+        python3) return 127 ;;
+    esac
+    return 0
+}
+"""
+    result = _run_pick_python(tmp_path, env_mock=env_mock)
+
+    assert result.returncode == 0
+    assert result.stdout.strip() == "python"
+
+
+def test_pick_python_fails_when_no_candidate_survives_env_probe(tmp_path):
+    env_mock = """
+env() { return 127; }
+"""
+    result = _run_pick_python(tmp_path, env_mock=env_mock)
+
+    assert result.returncode == 1
+    assert result.stdout.strip() == ""

scripts/serve.sh

@@ -42,7 +42,15 @@ fi
 _pick_python() {
     local candidate
     for candidate in python3 python py; do
-        if command -v "$candidate" >/dev/null 2>&1 && "$candidate" -c 'import sys; raise SystemExit(0 if sys.version_info.major >= 3 else 1)' >/dev/null 2>&1; then
+        # Probe through `env` as well: the frontend is launched as
+        # `env PORT=3000 "$DEERFLOW_PNPM_PYTHON" ...` (FRONTEND_CMD below), and on
+        # Windows/Git Bash the Microsoft Store python aliases under WindowsApps
+        # are skipped by Bash's own PATH lookup yet still resolved (and fail to
+        # exec) inside /usr/bin/env. A bare "$candidate" probe passes while the
+        # real launch dies with: env: 'python3': No such file or directory
+        if command -v "$candidate" >/dev/null 2>&1 \
+            && "$candidate" -c 'import sys; raise SystemExit(0 if sys.version_info.major >= 3 else 1)' >/dev/null 2>&1 \
+            && env "$candidate" -c 'import sys; raise SystemExit(0 if sys.version_info.major >= 3 else 1)' >/dev/null 2>&1; then
             printf '%s\n' "$candidate"
             return 0
         fi