fix: normalize LLM parameter case and improve type handling

- Add case normalization for 'LLM' parameter with deprecation warning
- Add comprehensive type conversion for LLM parameters
- Add proper error handling for parameter conversion
- Add tests to verify parameter handling

Fixes #1817

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

tests/test_agent.py

@@ -0,0 +1,63 @@
+import os
+import pytest
+from unittest import mock
+
+from crewai.agent import Agent
+from crewai.llm import LLM
+
+def test_agent_with_custom_llm():
+    """Test creating an agent with a custom LLM."""
+    custom_llm = LLM(model="gpt-4")
+    agent = Agent()
+    agent.role = "test"
+    agent.goal = "test"
+    agent.backstory = "test"
+    agent.llm = custom_llm
+    agent.allow_delegation = False
+    agent.post_init_setup()
+    
+    assert isinstance(agent.llm, LLM)
+    assert agent.llm.model == "gpt-4"
+
+def test_agent_with_uppercase_llm_param():
+    """Test creating an agent with uppercase 'LLM' parameter."""
+    custom_llm = LLM(model="gpt-4")
+    with pytest.warns(DeprecationWarning):
+        agent = Agent()
+        agent.role = "test"
+        agent.goal = "test"
+        agent.backstory = "test"
+        setattr(agent, 'LLM', custom_llm)  # Using uppercase LLM
+        agent.allow_delegation = False
+        agent.post_init_setup()
+    
+    assert isinstance(agent.llm, LLM)
+    assert agent.llm.model == "gpt-4"
+    assert not hasattr(agent, 'LLM')
+
+def test_agent_llm_parameter_types():
+    """Test LLM parameter type handling."""
+    env_vars = {
+        "temperature": "0.7",
+        "max_tokens": "100",
+        "presence_penalty": "0.5",
+        "logprobs": "true",
+        "logit_bias": '{"50256": -100}',
+        "callbacks": "callback1,callback2",
+    }
+    with mock.patch.dict(os.environ, env_vars):
+        agent = Agent()
+        agent.role = "test"
+        agent.goal = "test"
+        agent.backstory = "test"
+        agent.llm = "gpt-4"
+        agent.allow_delegation = False
+        agent.post_init_setup()
+        
+        assert isinstance(agent.llm, LLM)
+        assert agent.llm.temperature == 0.7
+        assert agent.llm.max_tokens == 100
+        assert agent.llm.presence_penalty == 0.5
+        assert agent.llm.logprobs is True
+        assert agent.llm.logit_bias == {50256: -100.0}
+        assert agent.llm.callbacks == ["callback1", "callback2"]