Fix CI failures on root — mock OSError instead of relying on filesystem

Tests assumed write to /nonexistent/... fails, but CI runs as root where
Linux allows creating any path. Use unittest.mock.patch on builtins.open
with OSError side_effect so the test is environment-independent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_ip_utils.py

@@ -5,6 +5,7 @@ import sys
 import os
 import tempfile
 import unittest
+import unittest.mock
 from pathlib import Path
 
 api_dir = Path(__file__).parent.parent / 'api'
@@ -98,7 +99,8 @@ class TestWriteEnvFile(unittest.TestCase):
         self.assertTrue(result)
 
     def test_returns_false_on_unwritable_path(self):
-        result = ip_utils.write_env_file('172.20.0.0/16', '/nonexistent/deep/path/.env')
+        with unittest.mock.patch('builtins.open', side_effect=OSError('Permission denied')):
+            result = ip_utils.write_env_file('172.20.0.0/16', '/any/path/.env')
         self.assertFalse(result)
 
     def test_contains_cell_network(self):