Fix test failures: improve CustomLLM and error handling

- Fix CustomLLM to handle structured output for guardrails with JSON response
- Add proper method implementations (supports_function_calling, etc.)
- Handle 'Thought:' pattern like working CustomLLM implementation
- Change invalid LLM test to use LiteAgent instead of Agent
- Improve error messages to use type() instead of __class__
- Address GitHub review feedback for better error handling

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

tests/test_lite_agent.py

@@ -433,8 +433,24 @@ def test_lite_agent_with_custom_llm_and_guardrails():
 
         def call(self, messages, tools=None, callbacks=None, available_functions=None, from_task=None, from_agent=None) -> str:
             self.call_count += 1
+            
+            if "valid" in str(messages) and "feedback" in str(messages):
+                return '{"valid": true, "feedback": null}'
+            
+            if "Thought:" in str(messages):
+                return f"Thought: I will analyze soccer players\nFinal Answer: {self.response}"
+            
             return self.response
 
+        def supports_function_calling(self) -> bool:
+            return False
+
+        def supports_stop_words(self) -> bool:
+            return False
+
+        def get_context_window_size(self) -> int:
+            return 4096
+
     custom_llm = CustomLLM(response="Brazilian soccer players are the best!")
     
     agent = Agent(
@@ -469,12 +485,14 @@ def test_lite_agent_with_custom_llm_and_guardrails():
 
 @pytest.mark.vcr(filter_headers=["authorization"])
 def test_lite_agent_with_invalid_llm():
-    """Test that Agent raises proper error with invalid LLM type."""
+    """Test that LiteAgent raises proper error with invalid LLM type."""
+    from crewai.lite_agent import LiteAgent
+    
     class InvalidLLM:
         pass
     
     with pytest.raises(ValueError) as exc_info:
-        Agent(
+        LiteAgent(
             role="Test Agent",
             goal="Test goal", 
             backstory="Test backstory",