[m-messer]

Ports the parsing half of compareExpressions#261 (big-operator custom symbols) to expression_parsing. The preview test stays in compareExpressions.

expression_parsing

• A task symbol whose LaTeX starts with a big operator (\sum, \int, \prod, \lim, \bigcup, \bigcap; BIG_OPERATOR_PREFIXES) wraps its argument when applied. With infsum as \sum_{n=1}^{\infty}, infsum((-1)^n/n^2) is an undefined function of the summand, and sympy_to_latex prints the operator followed by it. Before, it was the product infsum*(...), so the operator was printed inside the fraction or followed by \cdot.
• When it isn't applied (2*infsum), it is still the task's symbol.
• The function is an UndefinedFunction with a module-level _latex hook, so a config that uses one can still be pickled. #261 built a class around a closure, which can't be pickled.

Verification

• ruff, format and mypy are clean. The package tests pass, and so does the parity check (0 unexpected, 0 mismatched).

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