[Fix] Fix OlympiadBench grading of factored answers

[mkzung]

Motivation

MathJudger.is_equal runs four equivalence tests in sequence: interval, numerical,
expression, equation. The last three swallow a failure and fall through to the next
one. The interval test returns False, which ends the comparison.

is_interval classifies a string by its first and last character only, so a factored
product such as (x-1)(x+1) is taken for an interval. interval_equal strips the
outer brackets, leaving the fragment x-1)(x+1, and comparing it raises inside
compare_two_interval: expression_equal returns a sympy Relational when the
difference is still symbolic, and if not ... cannot evaluate it. The exception then
ends the whole comparison, so the expression test that proves these two answers
equal is never reached.

With sympy 1.14.0, OlympiadBenchEvaluator.score reports 0.0 accuracy for three such
prediction/reference pairs:

(x-1)(x+1)  vs (x^2-1)      is_interval=(True, True) interval_equal raised TypeError judge=False
(a+b)(a-b)  vs (a^2-b^2)    is_interval=(True, True) interval_equal raised TypeError judge=False
(x+2)(x+3)  vs (x^2+5x+6)   is_interval=(True, True) interval_equal raised TypeError judge=False
OlympiadBenchEvaluator accuracy: 0.0

Tightening is_interval so that a factored product is not classified as an interval
would keep these answers off the interval path in the first place. This PR does the
smaller thing: a failed interval test no longer ends the comparison.

Modification

• The interval handler in is_equal falls through to the remaining tests, as the
  numerical, expression and equation handlers already do.
• tests/datasets/test_olympiadbench.py covers a factored product against its
  expansion and against a wrong expansion, and pins interval comparison for equal,
  unequal and mixed-bracket bounds.

BC-breaking (Optional)

No. An answer that graded correct still grades correct; the change can only add a
later test, never remove one.

A sweep of 26 bracket-delimited pairs through MathJudger.judge moves three verdicts,
all of them false negatives:

'(x-1)(x+1)'  '(x^2-1)'      False -> True
'(a+b)(a-b)'  '(a^2-b^2)'    False -> True
'(x+2)(x+3)'  '(x^2+5x+6)'   False -> True

The other 23 are unchanged, including (1,2) vs (1,3), [0,1] vs (0,1),
(-\infty,0) vs (-\infty,0] and (x-1)(x+1) vs (x^2+1), which stay False, and
(0,\infty) vs (0,+\infty) and (\frac{1}{2},1) vs (0.5,1), which stay True.

Use cases (Optional)

Not applicable.

Tests

• pytest tests/datasets/test_olympiadbench.py -q — 2 passed. On main,
  test_factored_product_equals_expanded_form fails with
  AssertionError: False is not true and the interval test already passes.
• pytest tests/datasets tests/openicl tests/utils tests/evaluator --ignore=tests/datasets/test_local_datasets.py -q — 3 failed, 162 passed, 2 skipped.
  The same three test_livecodebench_evaluator cases fail unchanged on main
  (macOS multiprocessing spawn, unrelated to this PR).
• pre-commit run --files opencompass/datasets/OlympiadBench.py tests/datasets/test_olympiadbench.py — flake8, isort, yapf and codespell pass.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• CLA has been signed and all committers have signed the CLA in this PR.