Remove duplicate yield action_step in _run_stream()

[AhnJoonSung]

Fix duplicate action_step yield and NameError in _run_stream()

Description

This PR fixes two issues in the _run_stream method of the MultiStepAgent class:

1. When an agent reaches max_steps, the final action_step was being yielded twice - once in the finally block and again after the while loop
2. When max_steps=0, the code attempted to yield an undefined action_step variable, causing a NameError

Changes

• Removed the duplicate yield action_step on line 605 in src/smolagents/agents.py
• The action_step is already properly yielded in the finally block (line 600), so the additional yield was both redundant and problematic

Impact

This fix ensures that:

• Stream consumers receive each action_step exactly once
• Edge cases like max_steps=0 are handled gracefully without errors
• Both CodeAgent and ToolCallingAgent benefit from this fix since they inherit from MultiStepAgent

Fixes #1816

[aymeric-roucher]

HI @AhnJoonSung ! It's not really clear to me what was problematic before : could you add a test, or add assertions to an existing test, to show what was problematic before and now is solved + avoid regressions in the future?