Undo command does not undo markpaid all correctly

[nus-pe-bot]

Description: The undo command is said to "undo the most recent changes made to student data" in the User Guide. However, after using the markpaid command with all specified, undo only allows the user to unmark the last student as paid, rather than every student as expected. To further unmark every student as not having paid for the input month, undo has to be executed consecutively for each remaining student. Perhaps this could have been addressed more clearly in the User Guide so that the user is not misled?

Steps to reproduce:

1. Execute markpaid all m/2024-09
2. Execute undo
3. Note that only the last student's payment is unmarked, and all other students are still marked as having paid for the specified month.

Screenshots:

Before:

After:

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_{[original: nus-cs2103-AY2425S1/pe-interim#1969] [original labels: severity.Medium type.FunctionalityBug]}

[waihin26]

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