Function resolve_state 

pub fn resolve_state<S: Send + Sync + 'static>(
    states: &SharedStateMap,
) -> Result<Arc<S>, String>

Resolve a typed state from the shared map, returning an error string on failure.

This is used internally by stateful handlers. It is also re-exported for use by the erase_handler_with_state! / erase_handler_with_state_only! macros, which generate non-generic handler registration code.