[BUG] Fix in-place mutation in make_strings_unique

[SHIVANSH-ux-ys]

Fixes #551

The function make_strings_unique was mutating the input list in-place due to a direct reference assignment. This PR ensures a copy is created before modification.

Verified with a reproduction script:

mylist = ["a", "b", "a"]
make_strings_unique(mylist)
# mylist remains ["a", "b", "a"]

[fkiraly]

Great, thanks!

Can you add a test against regression? I.e., testing that the function no longer mutates its input?

[SHIVANSH-ux-ys]

Hi @fkiraly,

I have added a regression test in skbase/utils/tests/test_iter.py to ensure that make_strings_unique does not mutate its input lists (verifying both flat and nested list inputs).

The test passes successfully. Ready for another review!