Fix thinking config test isolation issues

Add environment isolation fixture to test_thinking_config_translation.py
to prevent THINKING_BUDGET environment variable pollution between tests.

All tests now pass: 3,044 passed, 38 skipped, 0 failed.

Author: Mateusz

tests/unit/test_thinking_config_translation.py

@@ -10,6 +10,21 @@
 from src.core.services.translation_service import TranslationService
 
 
+@pytest.fixture(autouse=True)
+def isolate_test_completely():
+    """Ensure complete test isolation by clearing any global state."""
+    import os
+
+    # Store original environment
+    original_env = dict(os.environ)
+
+    yield
+
+    # Restore original environment completely
+    os.environ.clear()
+    os.environ.update(original_env)
+
+
 class TestThinkingConfigTranslation:
     """Test reasoning_effort -> thinkingBudget translation."""