Standardized Idempotency-Key docs on journal and funding endpoints

Broker API

⚠️
Deprecation Warning
In the future, 128 character Idempotency-Key values will be deprecated in favor of 36 character UUIDs. Please preemptively migrate your keys to either UUIDv7 or UUIDv4, or any other <=36 character string.

The Idempotency-Key header is now described consistently across the idempotent Broker endpoints. The accepted contract is unchanged: the header remains optional and continues to accept keys up to 128 characters.

Uniform guidance to generate a unique UUIDv7 or UUIDv4 value (36 characters including hyphens) per logical operation, and to avoid reusing a key across operations.
Notes that Alpaca is moving toward a 36-character maximum, so new integrations should size keys accordingly.
POST /v1/journals keeps its documented idempotent replay behavior and now uses a shared description for the 422 response returned when a key is reused with a different request payload.

Affected Endpoints

POST /v1/instant_funding
POST /v1/journals
POST /v1/journals/batch