feat: add lifecycle hooks for agent execution flow

[wingding12]

Summary

Fixes #1883 - Add lifecycle hooks for CodeAgent execution flow

Adds lifecycle callbacks that fire at specific points during agent execution, enabling finer-grained control than step_callbacks (which only fire after steps complete). This addresses the use case of needing to execute callbacks at different points in the execution flow, such as sending LLM output to a TTS service before code execution.

New Lifecycle Events

Event	When it fires	Use cases
MODEL_OUTPUT	After LLM generates output, before parsing	TTS queuing, early logging, output validation
PRE_EXECUTION	Before code/tool execution, after parsing	Human-in-the-loop approval, custom code validation, security checks

Usage Example

from smolagents import CodeAgent, LifecycleEvent, ModelOutputEvent, PreExecutionEvent

def on_model_output(event: ModelOutputEvent, agent):
    # Send to TTS service before code execution starts
    tts_service.queue(event.model_output)

def on_pre_execution(event: PreExecutionEvent, agent):
    # Validate code before execution
    if "dangerous_function" in (event.code_action or ""):
        raise ValueError("Dangerous code blocked!")

agent = CodeAgent(
    tools=[...],
    model=model,
    lifecycle_callbacks={
        LifecycleEvent.MODEL_OUTPUT: on_model_output,
        LifecycleEvent.PRE_EXECUTION: on_pre_execution,
    }
)

Files Changed

File	Changes
src/smolagents/memory.py	Add LifecycleEvent enum, ModelOutputEvent/PreExecutionEvent dataclasses, LifecycleCallbackRegistry class
src/smolagents/agents.py	Add lifecycle_callbacks parameter to MultiStepAgent, trigger events in CodeAgent._step_stream and ToolCallingAgent._step_stream
tests/test_agents.py	Add 6 comprehensive tests for lifecycle hooks

Test plan

• Test setup of lifecycle callbacks registry
• Test invalid lifecycle event type raises error
• Test MODEL_OUTPUT callback in CodeAgent
• Test PRE_EXECUTION callback in CodeAgent (verifies order: model → callback → execution)
• Test lifecycle callbacks in ToolCallingAgent
• Test single-argument callback signature (backwards compatible)
• All existing tests still pass