fix: introduce sum_diff_eq_zero_of_equal_combinations

[JadAbouHawili]

No description provided.

[JadAbouHawili]

currently, sum_diff_eq_zero_of_equal_combinations is only mentioned in the hint and not present in the right side pane which the level directs the user towards to read about the necessary lemmas in level 7 of linear independence and span world

moreover , usage of (strict := true) here is a step in direction of resolving the repeated note:
"Note: This level may experience a hint display issue where hints repeat. If you see the same hint multiple times, the level is still working correctly - just continue with your proof as normal."

Judicious usage of (strict := true) for hints that suggest using obtain , by_cases , have or any other 'hypothesis introducing' tactic that doesn't change the goal will resolve repeated hint displays. You can refer to https://github.com/leanprover-community/lean4game/blob/main/doc/hints.md#5-strict-context-matching for some details

[ZRTMRH]

Thank you! Merged.