fix: add missing step3 field to CrashState and correct return value

[PrinceThummar011]

Description

Fixed two bugs in the fault tolerance example in 10_persistence.ipynb:

1. Missing state field: Added step3: str to CrashState TypedDict
2. Incorrect return value: Fixed step_3() function to return {"step3": "done"} instead of {"done": True}

Problem

The original code had inconsistent state management:

• CrashState defined step1 and step2 but was missing step3
• step_3() returned {"done": True} which doesn't match the state schema
• This would cause state tracking issues in the workflow

Solution

• Added step3: str field to maintain consistency with step1 and step2
• Changed return value to {"step3": "done"} to match the expected state structure
• Now all three steps follow the same pattern