New examples and descriptions across Broker API schemas

Broker API

Adds missing examples and descriptions to several Broker API schemas. Request and response shapes are unchanged.

• Adds a response example for the idempotency key conflict error on POST /v1/journals.
• Adds examples for the JITReportDownloadURL schema fields on the JIT reports endpoint.
• Adds a description and examples to the PortfolioHistoryError schema.
• Adds an example for account_sub_type and a request body description on account creation.
• Restores the deprecation notices on belongs_to and created_by in the admin action event schema, which carry a 2027-01-13 sunset; the duplicated deprecation text in their descriptions has been dropped so the notice stands alone.
• Simplifies redundant schema wrappers on account_type, account_sub_type, minor_identity, ACATS securities, and travel_rule_info, with no change to accepted or returned values.

Affected Endpoints

• POST /v1/journals
• POST /v1/accounts
• POST /v1beta1/acats/{account_id}
• GET /v2/events/admin-actions
• GET /v1/transfers/jit/reports
• GET /v1/trading/accounts/{account_id}/account/portfolio/history