Fix stale test assertions and truthsocial pytest dependency

- test_models: update xAI model expectations to grok-4-1-fast (matching
  current XAI_POLICY_MAP)
- test_openai_reddit: update fallback order assertion to gpt-4.1 (matching
  current MODEL_FALLBACK_ORDER)
- test_codex_auth: expect 'reddit' not 'web' when no API keys (Reddit
  is available via public JSON fallback)
- test_truthsocial: convert from pytest-style classes to unittest.TestCase,
  fix import path to use sys.path.insert pattern (matching all other tests)

tests/test_codex_auth.py

@@ -140,7 +140,8 @@ class TestGetAvailableSourcesWithAuth(unittest.TestCase):
             "XAI_API_KEY": None,
         }
         result = env.get_available_sources(config)
-        self.assertEqual(result, "web")
+        # Reddit is available via public JSON fallback even without OpenAI auth
+        self.assertEqual(result, "reddit")
 
 
 class TestParseCodexStream(unittest.TestCase):