feat: enhance LangGraphToolAdapter and improve conversion instructions

- Added type hints for better clarity and type checking in LangGraphToolAdapter.
- Updated conversion instructions to ensure compatibility with optional LLM checks.

src/crewai/agents/agent_adapters/langgraph/langgraph_tool_adapter.py

@@ -1,3 +1,8 @@
+from typing import Any, List, Optional
+
+from crewai.tools.base_tool import BaseTool
+
+
 class LangGraphToolAdapter:
     """Adapts CrewAI tools to LangGraph-compatible format"""
 

src/crewai/utilities/converter.py

@@ -216,7 +216,7 @@ def convert_with_instructions(
 
 def get_conversion_instructions(model: Type[BaseModel], llm: Any) -> str:
     instructions = "Please convert the following text into valid JSON."
-    if llm.supports_function_calling():
+    if llm and llm.supports_function_calling():
         model_schema = PydanticSchemaParser(model=model).get_schema()
         instructions += (
             f"\n\nOutput ONLY the valid JSON and nothing else.\n\n"