Function parse_funding_rate 

pub fn parse_funding_rate(
    msg: &DeriveTickerMsg,
    ts_init: UnixNanos,
) -> Result<Option<FundingRateUpdate>>

Parses a perpetual ticker payload into a [FundingRateUpdate].

Returns Ok(None) when the ticker does not carry funding.

Errors

Returns an error when the ticker timestamp is negative or overflows.