Allow partial credit in choices questions

[Copilot]

ChoiceQuestion and InlineMultiQuestion previously supported only binary correct/incorrect choices. This adds a correctness field to ChoiceDesc enabling fractional credit per choice.

Data model

• Replaced ChoiceMode.CORRECT/INCORRECT with a unified STANDARD mode plus a correctness: PointCount field (default 0.0)
• ~CORRECT~ remains backward-compatible (sets correctness=1.0); no-prefix choices continue to mean correctness=0.0
• Partial credit specified via YAML dict syntax:

choices:
  - ~CORRECT~ Fully correct answer
  - An incorrect answer
  - mode: standard
    correctness: 0.5
    text: A half-credit answer

Scoring changes

• ChoiceQuestion: grade() now returns choice.correctness directly instead of binary 0/1; page_correct_answer() displays the highest-correctness choice
• ChoicesAnswer (InlineMultiQuestion): get_correctness() returns choice.correctness directly

Validation

• ChoiceQuestion: at least one choice must have correctness > 0
• MultipleChoiceQuestion: partial credit (correctness ∉ {0, 1}) is disallowed via a new model validator

Tests

• Sandbox tests covering 0%, 50%, and 100% correctness for ChoiceQuestion
• PythonClassFakeRepo-based validation tests confirming partial-credit pages validate successfully and that invalid configurations (all-zero ChoiceQuestion, partial-credit MultipleChoiceQuestion) raise ValidationError