Add broker gateway facade and CLI commands

[AlstonLi007]

Summary

• add shared broker contracts to normalize order and position structures
• register IBKR and Tradovate gateways behind a broker routing facade and implement IBKR cancel
• extend the CLI with cancel, modify, and websocket listen commands for both brokers

Testing

• python -m openbb_lab.cli ibkr_ws_listen
• python -m openbb_lab.cli tradovate_ws_listen

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https://chatgpt.com/codex/tasks/task_e_68e7c7d8aa948321a86f599f5fcb2c7e

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Stream websocket events instead of materializing the iterator

The websocket listener handler converts broker_gateway.websocket_listen("ibkr") into a list and returns it. Broker websocket streams typically never terminate, so this call will block until the stream closes and will retain every event in memory before anything is printed. Iterating over the iterator and emitting each event as it arrives would allow the CLI command to display live updates and avoid unbounded memory growth. (The Tradovate handler mirrors the same pattern.)

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