Ban direct indexing in `Math.rs`

[JohnReedV]

Ban direct indexing in subtensor/src/math.rs

Problem:
The remaining #[allow(clippy::indexing_slicing)] live in math.rs, allowing potential panics in vec[idx] and slice[...].

Proposal:

1. Remove #[allow(clippy::indexing_slicing)] from pallets/subtensor/src/math.rs.
2. Replace every vec[idx] or slice[a..b] with a safe alternative, for example:

   // Before — can panic if `i >= vector.len()`
   idxs.sort_by_key(|&i| &vector[i]);
   
   // After — returns zero on OOB
   idxs.sort_by_key(|&i| {
       vector
           .get(i)
           .copied()
           .unwrap_or(I32F32::saturating_from_num(0))
   });```