Function extract_event 

pub fn extract_event<E: DeserializeOwned>(
    input: &DurableExecutionInvocationInput,
) -> Result<E, DurableExecutionInvocationOutput>

Extracts the user’s event from a DurableExecutionInvocationInput.

Tries these sources in order:

1. Top-level Input field (JSON value)
2. ExecutionDetails.InputPayload from the EXECUTION operation (JSON string)
3. null deserialization (for types with defaults, e.g. Option<T> or ())

Errors

Returns a DurableExecutionInvocationOutput with FAILED status if deserialization fails from all sources.