fix(sparse): floor() returns a copy instead of mutating its input (#740)

[SaguaroDev]

Closes #740.

chainladder.utils.sparse.floor(x) reassigned x.data in place, mutating the caller's COO array as a side effect:

def floor(x):
    x.data = np.floor(x.data)
    return x

Since np.floor() returns a copy, this makes floor() match that contract by copying before flooring, using the same .copy() idiom already present in nan_to_num() in the same module:

def floor(x):
    x = x.copy()
    x.data = np.floor(x.data)
    return x

Behavior now matches the issue's expectation:

a = array([1.2, 2.7, -0.3])
floor(a)
a.todense()        # array([ 1.2,  2.7, -0.3])  (unchanged)

a = floor(a)
a.todense()        # array([ 1.,  2., -1.])

The only internal caller (chainladder/core/correlation.py) already consumes the return value (m = xp.floor((n - 1) / 2)), so the change is behavior-preserving there.

Updated test_floor_mutates_in_place (which asserted result is a) to test_floor_returns_copy, asserting the result is a new object and the input is left unmodified. test_sparse.py and test_correlation.py pass (12 passed).

---

Note

Low Risk
Small API-contract fix in a utility helper with a single internal caller that already uses the return value; tests cover the new behavior.

Overview
chainladder.utils.sparse.floor no longer mutates the input COO array: it copies first (same pattern as nan_to_num), then floors x.data, aligning with np.floor’s non-mutating contract (issue #740).

Tests were renamed/updated from asserting in-place mutation (result is a) to asserting a new object and an unchanged input.

correlation.py already uses the return value only, so call sites are unchanged in practice.

^{Reviewed by Cursor Bugbot for commit 55c55e8. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.90%. Comparing base (8a9a46e) to head (55c55e8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #894   +/-   ##
=======================================
  Coverage   86.90%   86.90%           
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  Files          87       87           
  Lines        4932     4933    +1     
  Branches      624      624           
=======================================
+ Hits         4286     4287    +1     
  Misses        456      456           
  Partials      190      190           

Flag	Coverage Δ
unittests	86.90% <100.00%> (+<0.01%)	⬆️

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[henrydingliu]

since we are already touching this function, may I please trouble you to add type hinting for the input and output?