Add FinalAnswerStep to possible step_callbacks

[suryabdev]

Fix for #1879

There was a request to add FinalAnswerStep to the possible step_callbacks
We have final_answer_checks today that run at the end of a run (Which also receive agent memory as args), But the user said adding non validation logic to a final_answer_check overloaded the function
They also mentioned other reasons for adding it to the step_callbacks (See this issue comment for more details)

Reproduction code

from smolagents import CodeAgent, FinalAnswerStep, ActionStep, InferenceClientModel

def test_final_answer_callback(step: FinalAnswerStep, agent):
    """Callback that should be triggered when FinalAnswerStep is created."""
    print(f"Callback triggered for FinalAnswerStep: {step.output}")
    step.output = "MODIFIED BY CALLBACK"

# Create agent with step_callbacks
model = InferenceClientModel()
agent = CodeAgent(
    tools=[],
    model=model,  # your model instance
    step_callbacks={FinalAnswerStep: test_final_answer_callback},
)

# Run the agent
result = agent.run("What is 2+2?")
print(f"Agent run result: {result}") 

Output

╭─────────────────────────────────────────────────────────────────────────────── New run ───────────────────────────────────────────────────────────────────────────────╮
│                                                                                                                                                                       │
│ What is 2+2?                                                                                                                                                          │
│                                                                                                                                                                       │
╰─ InferenceClientModel - Qwen/Qwen3-Next-80B-A3B-Thinking ─────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Step 1 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
 ─ Executing parsed code: ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── 
  result = 2 + 2                                                                                                                                                         
  final_answer(result)                                                                                                                                                   
 ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── 
Final answer: 4
[Step 1: Duration 4.42 seconds| Input tokens: 2,031 | Output tokens: 401]
Callback triggered for FinalAnswerStep: 4
Agent run result: MODIFIED BY CALLBACK

[victorlearned]

Any traction here?