fix(xurl): treat PermissionError from PATH lookup as unavailable (#322)

is_available() only caught FileNotFoundError and TimeoutExpired. On WSL,
a /mnt/c/.../WindowsApps entry on $PATH returns EACCES during exec, and
Python raises PermissionError. That escaped is_available() and crashed
pipeline.diagnose() before any source ran.

Catch OSError instead. It covers FileNotFoundError, PermissionError, and
any other spawn-time OS error, so a non-executable xurl on PATH falls
through to the next backend instead of aborting the run.

tests/test_xurl_x.py

@@ -44,6 +44,13 @@ class TestIsAvailable(unittest.TestCase):
         with mock.patch("subprocess.run", side_effect=FileNotFoundError):
             self.assertFalse(xurl_x.is_available())
 
+    def test_returns_false_on_permission_error(self):
+        # WSL hits this when a Windows-mounted PATH entry points at an
+        # exec-blocked shim (e.g. WindowsApps), which raises PermissionError
+        # before any other PATH candidate is tried.
+        with mock.patch("subprocess.run", side_effect=PermissionError(13, "Permission denied", "xurl")):
+            self.assertFalse(xurl_x.is_available())
+
     def test_returns_false_on_timeout(self):
         import subprocess
         with mock.patch("subprocess.run", side_effect=subprocess.TimeoutExpired("xurl", 10)):