refactor(python): Mark `concat_arr` as unstable

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.44%. Comparing base (eaf9b3f) to head (22f68ab). Report is 22 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #19909 +/- ## ======================================= Coverage 79.43% 79.44% ======================================= Files 1554 1554 Lines 215610 215612 +2 Branches 2452 2452 ======================================= + Hits 171276 171292 +16 + Misses 43776 43762 -14 Partials 558 558 ```

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