Proxy: Fix memory load balancer server expiration handling

[liukun4515]

Summary

Fixes a critical issue in memory-based load balancer where streams could not recover when origin servers become unhealthy.

Changes

• Fixed MemoryLoadBalancer.Pick(): Now automatically switches to healthy servers when current server's UpdatedAt exceeds ServerAliveDuration (300s)
• Added double-checked locking: Ensures thread safety during concurrent server selection
• Added comprehensive unit tests: Verifies server expiration handling and concurrent access safety

Problem

Previously, once a stream was assigned to a server, it would remain bound to that server forever, even if the server became unhealthy (no heartbeat updates for >300s). This prevented streams from recovering when origin servers failed.

Solution

The Pick() method now:

1. Checks if previously picked server is still healthy (fast path, no lock)
2. If unhealthy, acquires lock and re-checks (double-checked locking pattern)
3. Selects a new healthy server if needed

Testing

• ✅ All unit tests pass
• ✅ Go race detector shows no race conditions
• ✅ Concurrent access tests verify thread safety (100 goroutines)
• ✅ Server expiration tests verify automatic failover

Files Changed

• proxy/internal/lb/mem.go: Added health checking and mutex-based locking
• proxy/internal/lb/mem_test.go: Added 5 comprehensive test cases (286 lines)

🤖 Generated with Claude Code

[liukun4515]

@winlinvip PTAL