JIT limits response schema corrected to match the API

Broker API

Updates the GET /v1/transfers/jit/limits response schema to match what the API actually returns:

Replaces daily_net_limit_in_use with in_use_limit, which reports real-time usage of the daily net limit.
Adds correspondent, executed_buys, executed_sells, open_buys, and open_sells.
Adds cash_held for options-enabled correspondents.
Clarifies the daily_net_limit description: further buying is halted once in_use_limit reaches this value.