Fix function_calling_llm support for custom models

- Add supports_function_calling() method to BaseLLM class with default True
- Add supports_function_calling parameter to LLM class to allow override of litellm check
- Add tests for both BaseLLM default and LLM override functionality
- Fixes #3708: Custom models not in litellm's list can now use function calling

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

tests/test_llm.py

@@ -711,3 +711,18 @@ def test_ollama_does_not_modify_when_last_is_user(ollama_llm):
     formatted = ollama_llm._format_messages_for_provider(original_messages)
 
     assert formatted == original_messages
+
+
+def test_supports_function_calling_with_override_true():
+    llm = LLM(model="custom-model/my-model", supports_function_calling=True)
+    assert llm.supports_function_calling() is True
+
+
+def test_supports_function_calling_with_override_false():
+    llm = LLM(model="gpt-4o-mini", supports_function_calling=False)
+    assert llm.supports_function_calling() is False
+
+
+def test_supports_function_calling_without_override():
+    llm = LLM(model="gpt-4o-mini")
+    assert llm.supports_function_calling() is True