[codex] Tighten REST retries and exchange factory

[createMonster]

Summary

• Implement RestClientConfig::max_retries for retryable GET/DELETE failures while intentionally avoiding automatic POST retries for order-style side effects.
• Route Bybit factory construction through the real Bybit builder and add OKX to ExchangeFactory.
• Update architecture docs to describe the current retry and factory behavior.

Why

The architecture audit found two concrete issues: retry configuration was documented but not applied, and ExchangeFactory bypassed builder behavior for Bybit while omitting the newly merged OKX connector.

Validation

• cargo test core::kernel::rest::tests --all-features -- --nocapture
• cargo test utils::exchange_factory::tests --all-features -- --nocapture
• cargo test --test binance_integration_tests binance_comprehensive_tests::test_error_handling_with_bad_credentials --all-features -- --nocapture
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features

Review Notes

A pre-PR read-only review found invalid mock Content-Length values in the new REST tests. The tests now derive Content-Length from the response body so they exercise the HTTP 500 mapping path directly.