Fix async provider reuse to prevent httpx loop-closed warnings

[RandomOscillations]

Summary

• route async simulation calls through cached simulation providers instead of creating a fresh provider instance per request
• allow get_simulation_provider to accept an optional model string so both Pass-1 and Pass-2 providers are cached/reused by provider
• add regression tests to ensure async calls use simulation cache (default and explicit model), while sync simple_call keeps existing provider-factory behavior

Why

Async simulation calls were bypassing provider cache and creating transient async SDK/httpx clients repeatedly. Those clients could be finalized during loop teardown and trigger Event loop is closed warnings.

Testing

• pytest tests/test_llm.py tests/test_providers.py -q
• pytest tests/test_engine.py tests/test_simulation.py tests/test_conversations.py -q

Closes #71