Isolate per-run logging for concurrent OpenEvolve instances

[00200200]

Summary

• Fixes Multiple Openevolve run calls in parallel result in clobbered loggers #295: concurrent OpenEvolve instances no longer stack unfiltered handlers on the root logger and cross-write into each other's log files.
• Handlers now attach to the openevolve package logger (propagate=False) and are gated by a per-run contextvars filter; handlers are cleaned up when run() finishes.

Test plan

• Added tests/test_parallel_logging_isolation.py (cross-write + cleanup regression)
• pytest tests/ -m "not slow and not integration" --ignore=tests/integration — 558 passed

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