Validate configured IBKR managed accounts

[Pigbibi]

Summary

• validate that configured IBKR account_ids are visible to the connected Gateway username before runtime work continues
• disconnect the probe/session immediately when the configured account is not available
• document the generic multi-linked-account Gateway pattern without private live config

Tests

• ./.venv/bin/python -m pytest tests/test_runtime_broker_adapters.py tests/test_event_loop.py::test_connect_ib_prepares_event_loop_before_connect tests/test_event_loop.py::test_connect_ib_retries_with_offset_client_ids tests/test_runtime_config_support.py::test_load_platform_runtime_settings_uses_account_group_secret -q