SplitContainer does not honor content ratio

### Describe the bug

I tried to implement a 1/3 - 2/3 UI with SplitContainer, by running this code inspired by the [Toga documentation](https://toga.readthedocs.io/en/stable/tutorial/tutorial-2.html):
```python
import toga
from toga.style.pack import COLUMN, Pack

class Tutorial2App(toga.App):

def startup(self):
data = [("root%s" % i, "value %s" % i) for i in range(1, 100)]

left_container = toga.Table(headings=["Hello", "World"], data=data)

right_content = toga.Box(style=Pack(direction=COLUMN, padding_top=50))

for b in range(0, 10):
right_content.add(
toga.Button(
"Hello world %s" % b,
style=Pack(width=200, padding=20),
)
)

right_container = toga.ScrollContainer(horizontal=False)

right_container.content = right_content

split = toga.SplitContainer()

# The content of the split container can be specified as a simple list:
# split.content = [left_container, right_container]
# but you can also specify "weight" with each content item, which will
# set an initial size of the columns to make a "heavy" column wider than
# a narrower one. In this example, the right container will be twice
# as wide as the left one.
split.content = [(left_container, 1), (right_container, 2)]

self.main_window = toga.MainWindow()
self.main_window.content = split
self.main_window.show()

def main():
return Tutorial2App("Tutorial 2", "org.beeware.toga.tutorial")

if __name__ == "__main__":
main().main_loop()
```

As a result, I get a 1/2 - 1/2 ratio (see screenshot)

### Steps to reproduce

Run the code above

### Expected behavior

Getting a 1/3 - 2/3 ratio

### Screenshots


### Environment

- Operating System: Ubuntu 24.04.1
- Python version: 3.12
- Toga version: 0.4.7

### Logs

```

```

### Additional context

_No response_

Reproduce the issue from the provided startup example using Toga 0.4.7, Python 3.12, and Ubuntu 24.04.1, then trace SplitContainer sizing from the `split.content = [(left_container, 1), (right_container, 2)]` entry point. Done means the example produces a 1/3–2/3 ratio rather than an even split.

Written by the indexing model from the issue text.