Five more resources following clients.rs's template exactly, same as
the previous batch. credits.rs is otherwise identical to quotes.rs
since Credit is also a type alias for Invoice in the SDK (same table,
distinguished by entity_type upstream).
Verified against a live local server: full create/list/get/update/
delete round trip for all five.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
