fix: return PortError for read timeouts in Unix and Windows serial ports

[uamana]

Explicit ReadTimeout Error for bufio.Reader Support

Summary

This PR ensures that read timeout errors are explicitly returned as PortError{code: ReadTimeout}, enabling proper timeout tracking.

Example Usage

port, _ := serial.Open("/dev/ttyUSB0", &serial.Mode{})
port.SetReadTimeout(1 * time.Second)
reader := bufio.NewReader(port)

for {
    line, err := reader.ReadLine()
    if err != nil {
        if portErr, ok := err.(*serial.PortError); ok {
            if portErr.Code() == serial.ReadTimeout {
                // Timeout occurred - check for buffered data
                if reader.Buffered() > 0 {
                    continue // Process buffered data
                }
                // No data available, timeout is expected
                continue
            }
        }
        // Other error occurred
        return err
    }
    // Process line
}

Related

This change improves integration with bufio.Reader by making timeout errors explicitly identifiable. Without it bufio.Reader returns error about multiple empty reads.