Handle new API field per 2025-12-02 update.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Lol, a cheeky unforeseen bug due to TOML's lack of a null type and
thinking i can render an `Optional` field on a `msgspec.Struct`
(defaulted to `None`) the `binance.symcache.toml` cache file..
I didn't catch this when i first updated to the 3.1 API in f7caa75228
because i never did a cache-files flush.. lesson learned and we **really
need tests for this**!!
Such that the next time i inevitably must debug the some order-request
error status or precision discrepancy, i have the mkt-symbol branch
ready to go. Also, switch to `'action': 'buy'|'sell' as action,` style
`case` matching instead of the post-`if` predicate style.
Tyler Goodlet