Improve expected format hints/parsing

In order to bypass limitations of the sympy LaTeX parser, some typing hints (*Expected format*) were shown before the answer area. This seems not to help students very much.

Hints or parsing should be improved. Hints could probably be improved by being shown in the formatted way, so that the students could compare graphically the expected format with the one they typed. Typical parser limitations typed by students could also be caught in order to suggest improving the formatting.

Other possibilities are to investigate solving parsing issues upstream by identifying which valid LaTeX typings that aren't well supported by the sympy parser and that can be useful for the whole sympy community, and contribute.

The issue names no files, tests, or entry points. Start by locating the expected-format hint and SymPy LaTeX parsing flow, then review how student input is displayed and which valid LaTeX forms fail. Done means the scope is agreed and either clearer formatted hints, targeted parser feedback, or an upstream SymPy issue is implemented and tested.

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