tests: skip POSIX-venv-layout tests on Windows

test_code_execution_modes.py had two test-level failures and two
class-level stale skip reasons on this Windows-native branch:

  - TestResolveChildPython::test_project_with_virtualenv_picks_venv_python
  - TestResolveChildPython::test_project_prefers_virtualenv_over_conda

Both fail on Windows with OSError: [WinError 1314] — they call
pathlib.Path.symlink_to() to build a fake venv, which requires
developer mode or admin on Windows.  They also assume POSIX venv
layout (bin/python) where Windows uses Scripts/python.exe.  Skip
them with a specific, accurate reason.

Also updated two class-level skipif reasons that said
'execute_code is POSIX-only' — no longer true on this branch.
New reason explains it's the test infrastructure (symlinks + POSIX
venv layout) that's the blocker, not execute_code itself.

Results on Windows Python 3.11:
  Before: 41 passed, 10 skipped, 2 failed
  After:  43 passed, 12 skipped, 0 failed

tests/tools/test_code_execution_modes.py

@@ -131,6 +131,12 @@ class TestResolveChildPython(unittest.TestCase):
 
     def test_project_with_virtualenv_picks_venv_python(self):
         """Project mode + VIRTUAL_ENV pointing at a real venv → that python."""
+        if sys.platform == "win32":
+            pytest.skip(
+                "Creates symlinks and assumes POSIX venv layout (bin/python). "
+                "Windows venvs use Scripts/python.exe and symlink creation "
+                "requires elevated privileges (WinError 1314)."
+            )
         import tempfile, pathlib
         with tempfile.TemporaryDirectory() as td:
             fake_venv = pathlib.Path(td)
@@ -154,6 +160,12 @@ class TestResolveChildPython(unittest.TestCase):
 
     def test_project_prefers_virtualenv_over_conda(self):
         """If both VIRTUAL_ENV and CONDA_PREFIX are set, VIRTUAL_ENV wins."""
+        if sys.platform == "win32":
+            pytest.skip(
+                "Creates symlinks and assumes POSIX venv layout (bin/python). "
+                "Windows venvs use Scripts/python.exe and symlink creation "
+                "requires elevated privileges (WinError 1314)."
+            )
         import tempfile, pathlib
         with tempfile.TemporaryDirectory() as ve_td, tempfile.TemporaryDirectory() as conda_td:
             ve = pathlib.Path(ve_td)
@@ -257,7 +269,15 @@ class TestModeAwareSchema(unittest.TestCase):
 # Integration: what actually happens when execute_code runs per mode
 # ---------------------------------------------------------------------------
 
-@pytest.mark.skipif(sys.platform == "win32", reason="execute_code is POSIX-only")
+@pytest.mark.skipif(
+    sys.platform == "win32",
+    reason=(
+        "Assumes POSIX venv layout (bin/python) and symlink creation "
+        "privileges.  execute_code itself works on Windows — these "
+        "integration tests just haven't been ported to the Scripts/"
+        "python.exe layout yet."
+    ),
+)
 class TestExecuteCodeModeIntegration(unittest.TestCase):
     """End-to-end: verify the subprocess actually runs where we expect."""
 
@@ -351,7 +371,15 @@ class TestExecuteCodeModeIntegration(unittest.TestCase):
 # changes CWD + interpreter, not the security posture.
 # ---------------------------------------------------------------------------
 
-@pytest.mark.skipif(sys.platform == "win32", reason="execute_code is POSIX-only")
+@pytest.mark.skipif(
+    sys.platform == "win32",
+    reason=(
+        "Assumes POSIX venv layout (bin/python) and symlink creation "
+        "privileges.  execute_code itself works on Windows — these "
+        "integration tests just haven't been ported to the Scripts/"
+        "python.exe layout yet."
+    ),
+)
 class TestSecurityInvariantsAcrossModes(unittest.TestCase):
 
     def _run(self, code, mode):