fix: remove duplicate action_step yield and prevent NameError when max_steps=0

[gauravSsinha]

Description

Fixes two bugs in MultiStepAgent._run_stream():

1. Duplicate yield: When max_steps is reached, action_step was yielded in the finally block AND again after the while loop. Stream consumers received duplicate step events.

2. NameError when max_steps=0: If max_steps=0, the while loop never executes, so action_step is never defined. The post-loop yield action_step raises NameError.

Fix

Remove the redundant yield action_step after the while loop. The finally block already yields every action_step on each iteration, and _handle_max_steps_reached() handles the max-steps case without needing to re-yield.

One-line change

         if not returned_final_answer and self.step_number == max_steps + 1:
             final_answer = self._handle_max_steps_reached(task)
-            yield action_step

Fixes #1816