help: two-item columns are broken since #516

**Describe the bug**

#516 fixed one bug but exposed another subtle one that shows itself when the help bubble is configured to show columns that each contain two items. In the demos below, I'm using `xxxxxx` as the `FullSeparator` so it's more apparent what is different between the before/after:

Behavior before the fix in #516


Behavior after the fix in #516


So the fix the issue of extra padding on the right side, but now only the first column uses the `FullSeparator`.

**Setup**
- macOS
- zsh
- Ghostty
- en_US.UTF-8

**To Reproduce**

Reproduction script, based on the help example in charmbracelet/bubbletea

```go
package main

import (
"fmt"
"os"
"strings"

"github.com/charmbracelet/bubbles/help"
"github.com/charmbracelet/bubbles/key"
tea "github.com/charmbracelet/bubbletea"
"github.com/charmbracelet/lipgloss"
)

// keyMap defines a set of keybindings. To work for help it must satisfy
// key.Map. It could also very easily be a map[string]key.Binding.
type keyMap struct {
Up key.Binding
Down key.Binding
Left key.Binding
Right key.Binding
Help key.Binding
Quit key.Binding
}

// ShortHelp returns keybindings to be shown in the mini help view. It's part
// of the key.Map interface.
func (k keyMap) ShortHelp() []key.Binding {
return []key.Binding{k.Help, k.Quit}
}

// FullHelp returns keybindings for the expanded help view. It's part of the
// key.Map interface.
func (k keyMap) FullHelp() [][]key.Binding {
return [][]key.Binding{
{k.Up, k.Down},
{k.Left, k.Right},
{k.Up, k.Down},
{k.Left, k.Right},
{k.Help, k.Quit},
}
}

var keys = keyMap{
Up: key.NewBinding(
key.WithKeys("up", "k"),
key.WithHelp("↑/k", "move up"),
),
Down: key.NewBinding(
key.WithKeys("down", "j"),
key.WithHelp("↓/j", "move down"),
),
Left: key.NewBinding(
key.WithKeys("left", "h"),
key.WithHelp("←/h", "move left"),
),
Right: key.NewBinding(
key.WithKeys("right", "l"),
key.WithHelp("→/l", "move right"),
),
Help: key.NewBinding(
key.WithKeys("?"),
key.WithHelp("?", "toggle help"),
),
Quit: key.NewBinding(
key.WithKeys("q", "esc", "ctrl+c"),
key.WithHelp("q", "quit"),
),
}

type model struct {
keys keyMap
help help.Model
inputStyle lipgloss.Style
lastKey string
quitting bool
}

func newModel() model {
h := help.New()
h.FullSeparator = "xxxxxx" // using visible characters to clearly demonstrate the problem
return model{
keys: keys,
help: h,
inputStyle: lipgloss.NewStyle().Foreground(lipgloss.Color("#FF75B7")),
}
}

func (m model) Init() tea.Cmd {
return nil
}

func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
switch msg := msg.(type) {
case tea.WindowSizeMsg:
// If we set a width on the help menu it can gracefully truncate
// its view as needed.
m.help.Width = msg.Width

case tea.KeyMsg:
switch {
case key.Matches(msg, m.keys.Up):
m.lastKey = "↑"
case key.Matches(msg, m.keys.Down):
m.lastKey = "↓"
case key.Matches(msg, m.keys.Left):
m.lastKey = "←"
case key.Matches(msg, m.keys.Right):
m.lastKey = "→"
case key.Matches(msg, m.keys.Help):
m.help.ShowAll = !m.help.ShowAll
case key.Matches(msg, m.keys.Quit):
m.quitting = true
return m, tea.Quit
}
}

return m, nil
}

func (m model) View() string {
if m.quitting {
return "Bye!\n"
}

var status string
if m.lastKey == "" {
status = "Waiting for input..."
} else {
status = "You chose: " + m.inputStyle.Render(m.lastKey)
}

helpView := m.help.View(m.keys)
height := 8 - strings.Count(status, "\n") - strings.Count(helpView, "\n")

return "\n" + status + strings.Repeat("\n", height) + helpView
}

func main() {
if os.Getenv("HELP_DEBUG") != "" {
f, err := tea.LogToFile("debug.log", "help")
if err != nil {
fmt.Println("Couldn't open a file for logging:", err)
os.Exit(1)
}
defer f.Close() // nolint:errcheck
}

if _, err := tea.NewProgram(newModel()).Run(); err != nil {
fmt.Printf("Could not start program :(\n%v\n", err)
os.Exit(1)
}
}
```

~~I'm working on a fix for this right now!~~