fix(check): windows pnpm version detection in check script (#2189)

* fix: resolve Windows pnpm detection in check script

* style: format check script regression test

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix: resolve corepack fallback on windows

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

backend/tests/test_check_script.py

@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+import importlib.util
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+CHECK_SCRIPT_PATH = REPO_ROOT / "scripts" / "check.py"
+
+
+spec = importlib.util.spec_from_file_location("deerflow_check_script", CHECK_SCRIPT_PATH)
+assert spec is not None
+assert spec.loader is not None
+check_script = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(check_script)
+
+
+def test_find_pnpm_command_prefers_resolved_executable(monkeypatch):
+    def fake_which(name: str) -> str | None:
+        if name == "pnpm":
+            return r"C:\Users\tester\AppData\Roaming\npm\pnpm.CMD"
+        if name == "pnpm.cmd":
+            return r"C:\Users\tester\AppData\Roaming\npm\pnpm.cmd"
+        return None
+
+    monkeypatch.setattr(check_script.shutil, "which", fake_which)
+
+    assert check_script.find_pnpm_command() == [r"C:\Users\tester\AppData\Roaming\npm\pnpm.CMD"]
+
+
+def test_find_pnpm_command_falls_back_to_corepack(monkeypatch):
+    def fake_which(name: str) -> str | None:
+        if name == "corepack":
+            return r"C:\Program Files\nodejs\corepack.exe"
+        return None
+
+    monkeypatch.setattr(check_script.shutil, "which", fake_which)
+
+    assert check_script.find_pnpm_command() == [
+        r"C:\Program Files\nodejs\corepack.exe",
+        "pnpm",
+    ]
+
+
+def test_find_pnpm_command_falls_back_to_corepack_cmd(monkeypatch):
+    def fake_which(name: str) -> str | None:
+        if name == "corepack":
+            return None
+        if name == "corepack.cmd":
+            return r"C:\Program Files\nodejs\corepack.cmd"
+        return None
+
+    monkeypatch.setattr(check_script.shutil, "which", fake_which)
+
+    assert check_script.find_pnpm_command() == [
+        r"C:\Program Files\nodejs\corepack.cmd",
+        "pnpm",
+    ]