modern-treasury python: Idempotency-Key passed via extra_headers gets overwritten
## Problem
Setting a custom idempotency key via `extra_headers={'Idempotency-Key': 'mykey123'}` on e.g. `client.payment_orders.create(...)` didn't stick: framework code looked for `options.idempotency_key` and overwrote the header with its own generated `self._idempotency_key()`. A maintainer confirmed this was a bug. The reporter's interim workaround was to assign `client._idempotency_key` to a custom function after creating the client.
## Verified fix
Confirmed bug: the SDK ignored a manually provided Idempotency-Key header and replaced it with an auto-generated one. The maintainer said the fix would add an `idempotency_key` kwarg and also honor a manually provided header; the fix was merged and released in 1.10. Until you can upgrade, the reporter's workaround is to override `client._idempotency_key` with your own function after constructing the client. Source: https://github.com/Modern-Treasury/modern-treasury-python/issues/70