fix: round the displayed problem score to two decimal places

[Agrendalath]

When a problem's weight doesn't divide evenly among its questions, the weighted score is a repeating fraction and is rendered in full. I.e., the learners see 0.3333333333333333/1.0 point. However, the notification that appears when learners submit the answer is already rounded to two decimal places. This PR changes the main score message to match this behavior.

Before

After

Testing instructions

Create the following Problem XBlock and test how the grade is displayed:

<problem>
<optionresponse><div>Q1</div><optioninput><option correct="true">True</option><option correct="false">False</option><option correct="false">False</option></optioninput></optionresponse>
<optionresponse><div>Q2</div><optioninput><option correct="true">True</option><option correct="false">False</option><option correct="false">False</option></optioninput></optionresponse>
<optionresponse><div>Q3</div><optioninput><option correct="true">True</option><option correct="false">False</option><option correct="false">False</option></optioninput></optionresponse>
</problem>

Author's notes

1. I was wondering whether we should use str(progress.frac()) if progress else (0, 0), but using the logic from the __str__ method felt somewhat hacky.
2. I intentionally omitted rounding total, as changing 1.0 to 1 in this line could be considered a refactor that requires a product decision, instead of being a simple fix.

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