Move the Tool calling to Step Scope

Currently the Tool Calls are happening in the session scope and we are maintaining the tools, deferred tools all in the session. As the certain tool call is specific to a certain step, i believe it should be happening in the scope of the Step.

So when the we generate the next decision we will only say `self.current_step.get_next_decision_model(decision_constraints)` doing so the `self.current_step` should be changed if needed and if there is a step transition is happening or after a successful tool call or successful responding, the `current step` should be resetted to the original state.

From the users point of view there should be no change whatsoever.