Fix/sum preview

[m-messer]

Problem:
Custom symbols defined with big-operator LaTeX (e.g. infsum with \sum_{n=1}^{\infty}) were treated as plain SymPy Symbol objects. This meant infsum((-1)^n / n^2) parsed as multiplication (Symbol("infsum") * ((-1)^n / n^2)), causing the sum symbol to appear in fraction numerators or with spurious \cdot multiply dots in the preview. For example, from JF's course:

• infsum((-1)^n / n^2) rendered with the sum symbol inside the fraction numerator instead of wrapping the expression
• pi^2/3 + infsum((-1)^n*cos(n*x)*(4/n^2) rendered with a \cdot immediately after the sum symbol

Solution:
Detect prefix operator symbols automatically from their existing latex definition: if a symbol's LaTeX starts with a big operator command (\sum, \int, \prod, \lim, \bigcup, \bigcap), create a SymPy Function subclass for it instead of a Symbol. No changes to teacher-facing configuration are required.

A SymPy Function subclass is necessary because of how SymPy's parser handles callable objects in local_dict. When infsum is a Symbol, SymPy's auto-multiplication transformation converts infsum(x) into Symbol("infsum") * x.

By placing a Function subclass in local_dict instead, SymPy's parser treats infsum(x) as a genuine function application InfSum(x), the subclass carries a _latex() method that the Sy
mPy printer calls when it encounters InfSum(x), returning
{symbol_latex} {arg_latex} directly.

Changes:

• app/utility/expression_utilities.py - added BIG_OPERATOR_PREFIXES constant; modified sympy_symbols() to create a dynamic Function subclass (with custom _latex()) for any symbol whose LaTeX begins with a big operator prefix, falling back to Symbol otherwise.
• app/preview_test.py - added test_sum_preview

To test please use the sandbox here