fix: fix misc bugs

[tlambert03]

1. process_request squeeze removed singleton visible axes — .squeeze() stripped all size-1 dimensions, including visible axes. Replaced with a targeted .reshape(vis_shape) that only collapses non-visible, non-channel dims.

2. Negative current_index values produced empty slices — _norm_current_index only clamped values exceeding max_val, so -1 passed through and became slice(-1, 0) (empty). Added clamping for val < 0.

3. _calc_hist_bins crashed on float data — np.iinfo(data.dtype) raises for float dtypes. Now falls back to np.histogram for non-integer data

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.96%. Comparing base (9df4125) to head (8620ac7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
- Coverage   87.05%   86.96%   -0.10%     
==========================================
  Files          46       46              
  Lines        5330     5338       +8     
==========================================
+ Hits         4640     4642       +2     
- Misses        690      696       +6     

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