feat: improve connection safety and add dial timeout support

[yudhasubki]

Summary

This PR improves connection pool safety, adds dial timeout support, and modernizes context cancellation handling.

Changes

🚀 New Features

• Dial Timeout: Added WithDialTimeout(d time.Duration) option to prevent indefinite blocking when dialing unresponsive servers
  • New ErrDialTimeout error when dial exceeds configured timeout
  • Proper cleanup of late-completing dial operations

🛡️ Safety Improvements

• Added nil-check protection in Read() and Write()(cci:1://file:///Users/yudhapratamaputra/Project/yudhasubki/netpool/pool_connection.go:97:0-113:1) to prevent race condition panics
• Override SetDeadline, SetReadDeadline, SetWriteDeadline, LocalAddr, RemoteAddr methods with proper closed-connection checks
• Auto-reset deadline in Close() before returning connection to pool (prevents deadline leakage to next user)
• Fixed MarkUnusable() to properly return object to sync.Pool

netpool.go:

• Replaced goroutine+channel pattern with context.AfterFunc (Go 1.21+) for cleaner context cancellation handling

Usage Example

pool, err := netpool.New(dialFn,
    netpool.WithMinPool(5),
    netpool.WithMaxPool(50),
    netpool.WithDialTimeout(5*time.Second),
)