ProxyPool replaces ProxySource with:
- Multiple sources: HTTP APIs and text files (one proxy URL per line)
- Deduplication by proto://host:port
- Health testing: full chain test with configurable concurrency
- Mass-failure guard: skip eviction when >90% fail
- Background loops for periodic refresh and health checks
- JSON state persistence with atomic writes (warm starts)
- Backward compat: ProxySource still works for legacy configs
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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