Propagate PaperBench judge validity through aggregate nodes

[sylvesterkaczmarek]

Summary

Make aggregate PaperBench judge nodes reflect grading failures in their descendants instead of always claiming a valid score.

Leaf/subtree grading exceptions are already converted to GradedTaskNode values with valid_score=False, but every internal node built after recursively grading children is currently hard-coded to valid_score=True. The final root can therefore appear valid while containing one or more judge failures.

Fixes #144.

Fix

Set aggregate validity from the children:

valid_score = all(child.valid_score for child in graded_sub_tasks)

The weighted score calculation is unchanged.

Regression coverage

Adds async tests verifying that:

• one successful child plus one child whose grading raises produces the same weighted score but an invalid aggregate/root grade;
• the successful child remains valid while the failed child is invalid;
• an all-valid tree still produces valid_score=True.

No grading prompts, exception handling, score weights, or leaf-grade semantics are changed.