Add live paper and processing pool tests for contingent orders

[Martin-Molinero]

Follow up of #9828: unit tests for the live paper trading path of contingent orders, which the existing tests cover in backtesting mode only, and a sturdier brokerage trigger test.

Test	Covers
BacktestingTransactionHandlerTests.ProcessesContingentOrdersInLivePaperTrading	a bracket through the paper brokerage in live mode with concurrency on: placed by the transaction threads once the set arrived, children held, triggered by the entry fill, take profit fills and cancels the stop loss
OrderRequestProcessingPoolTests.ContingentOrdersAreProcessedInArrivalOrderWithoutBlockingOtherOrders	the orders of a set are processed one at a time in arrival order, an unrelated order doesn't wait for them, set ids don't collide with order ids
BrokerageTests.ContingentOrdersTrigger	now cancels the triggered orders and waits for them to be canceled (30s timeout) before the tear down closes the position: Alpaca rejected that close while the take profit still reserved the shares

The two new unit tests run in under 100 ms.

🤖 Generated with Claude Code