fix: resolve CI failures for fallback LLM implementation

- Fix type checker error by adding None check before raising last_exception
- Fix ContextWindowExceededError constructor with correct signature (message, model, llm_provider)
- Update auth error test assertion to match new print message format

Co-Authored-By: João <joao@crewai.com>

tests/agent_test.py

@@ -1984,7 +1984,7 @@ def test_crew_agent_executor_litellm_auth_error():
         )
 
     # Verify error handling messages
-    error_message = f"Error during LLM call: {str(mock_llm_call.side_effect)}"
+    error_message = f"Primary LLM failed: {str(mock_llm_call.side_effect)}"
     mock_printer.assert_any_call(
         content=error_message,
         color="red",