Implement get_input_schema(), get_output_schema(), get_graph() for StateGraph, MessageGraph

[nfcampos]

Depends on https://github.com/langchain-ai/langchain/pull/17745

Final step will be to add a graphviz-based draw method (like https://github.com/langchain-ai/langgraph/pull/71) in langchain-core

Produces graphs like this

                       +-----------+            
                       | __start__ |            
                       +-----------+            
                             *                  
                             *                  
                             *                  
                         +-------+              
                         | agent |              
                        *+-------+*             
                      **           ***          
                    **                **        
                  **                    **      
   +-----------------------+              **    
   | agent_should_continue |               *    
   +-----------------------+               *    
               *            ****           *    
               *                *****      *    
               *                     ***   *    
          +---------+                 +-------+ 
          | __end__ |                 | tools | 
          +---------+                 +-------+