Use gepa async evaluator support in GEPA example

[benediktstroebl]

Simplifies the GEPA example by leveraging gepa's new async evaluator support (gepa-ai/gepa#278):

• Make evaluate/run_trial async so gepa auto-detects them and uses asyncio.gather() for batch concurrency
• Remove the manual background event loop thread and run_coroutine_threadsafe bridging
• Remove max_workers from EngineConfig (concurrency now controlled by TrialQueue's semaphore)
• Pin gepa to feat/async-evaluator-support branch