[#3] Fix connection storm: prevent MaxConns violations during DB recovery

[lincai505011-ops]

/claim #3

What

Fix connection storm in pgxpool.Pool.Acquire() that allowed MaxConns violations during database recovery.

Root Cause

The original code used len(p.conns) to check pool capacity, but this only counts idle connections currently in the pool — not connections that have been checked out. During DB recovery, multiple goroutines exiting waitForConn would all observe an empty pool and simultaneously initiate connection creation, exceeding MaxConns.

Fix

• Added totalConns int32 (atomic) to track ALL connections: idle + in-use + in-flight
• Acquire() now checks totalConns + inFlightConns >= maxConns before allowing new connection creation
• inFlightConns is properly decremented on dial failure to prevent pool starvation
• Added sync.Cond for efficient blocking when pool is at capacity
• Added comprehensive test coverage

Tests (all passing)

• TestMaxConnsNotExceeded — 50 concurrent goroutines, maxConns=5, never exceeded
• TestRecoveryAfterFailure — dial failures correctly decrement counters, subsequent acquire succeeds
• TestInFlightCounterDecrementedOnFailure — all-failure scenario resets counters to 0
• TestConnectionStormPrevention — recovery burst never exceeds maxConns (max 5 concurrent dials)
• TestContextCancellation — cancelled context returns error
• TestPoolClosed — closed pool returns ErrPoolClosed

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