expression-parsing: read sum(expr) without limits as an unevaluated sum

[m-messer]

Fixes a regression found in the compareExpressions refactor sweep. With elementary_functions on, sum(...) with no limits was rewritten to SymPy's summation, which needs limits. Every submission to an affected question then got an HTTP 500, correct answers included.

Changes

• One-argument sum(expr) now parses to UNEVALUATED_SUM(expr), an undefined function sum. Two sums are equal when their summands simplify equal. Nothing is factored out, because the index is unknown.
• sum(f, (k, a, b)) and summation(...) still evaluate.
• An applied sum(...) is a sum even when the task defines sum as a symbol code or alias. A bare sum stays the task's symbol (ALWAYS_FUNCTIONS in symbol_tables.py).
• sympy_to_latex prints it as \sum{\left(… \right)}.
• CHANGELOG entry under 0.3.0 Fixed.

Behaviour change: old cE only graded these questions because, by accident, it read sum as a plain symbol. Report case 3e65804e now grades incorrect, because its reference answer has sum(1/(2k+1)) * sin(...), with sin outside the sum. That reference answer needs fixing.

Testing: new TestSumWithoutLimits in tests/test_regressions.py. The expression_parsing, criteria and units suites pass, and ruff and mypy are clean. I also ran the report's repro cases through the cE refactor branch with this package: no 500s.

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