Add IBKR execution backend routing config

[Pigbibi]

Summary

• add account-group execution_backend routing for gateway vs quantconnect
• keep gateway as the default and preserve existing Gateway requirements
• allow quantconnect account groups to omit Gateway host/port/client id while fail-fast protecting direct Gateway execution
• surface backend metadata in runtime logs/reports and document placeholder-only QuantConnect fields
• update QuantPlatformKit pin to include the shared QuantConnect connector framework

Verification

• timeout 180 .venv/bin/python -m pytest -q
• ruff check runtime_config_support.py main.py scripts/build_cloud_run_env_sync_plan.py scripts/print_strategy_switch_env_plan.py tests/test_runtime_config_support.py tests/test_event_loop.py tests/test_connect_timeout_alert.py

Notes

This does not place orders through QuantConnect yet. It creates the platform routing boundary so a later QuantConnect algorithm/handoff path can be added without changing strategy code or Gateway behavior.