fix: DataFrame.rows_by_key returning key tuples with elements in wrong order

[lukemanley]

Fixes the following and simplifies the implementation a bit.

In [1]: import polars as pl

In [2]: df = pl.DataFrame(
   ...:     {
   ...:         "w": ["a", "b", "b", "a"],
   ...:         "x": ["q", "q", "q", "k"],
   ...:         "y": [1.0, 2.5, 3.0, 4.5],
   ...:         "z": [9, 8, 7, 6],
   ...:     }
   ...: )

In [3]: df.rows_by_key(["x", "w"])
Out[3]: 
defaultdict(list,
            {('a', 'q'): [(1.0, 9)],   # <-- the key tuples are ordered as (column w, column x), not (x, w) as requested
             ('b', 'q'): [(2.5, 8), (3.0, 7)],
             ('a', 'k'): [(4.5, 6)]})

In [4]: df.rows_by_key(["x", "w"], named=True)
Out[4]: 
defaultdict(list,
            {('q', 'a'): [{'y': 1.0, 'z': 9}],   # <-- the key tuples are ordered correctly as (x, w)
             ('q', 'b'): [{'y': 2.5, 'z': 8}, {'y': 3.0, 'z': 7}],
             ('k', 'a'): [{'y': 4.5, 'z': 6}]})

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.99%. Comparing base (e7a84bc) to head (8aaff53).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19486      +/-   ##
==========================================
+ Coverage   79.94%   79.99%   +0.05%     
==========================================
  Files        1534     1536       +2     
  Lines      211183   211240      +57     
  Branches     2444     2435       -9     
==========================================
+ Hits       168831   168990     +159     
+ Misses      41797    41696     -101     
+ Partials      555      554       -1     

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[alexander-beedie]

Thanks; I'll take a look at this one later tonight (or tomorrow).
Looks much simplified indeed 👍

[lukemanley]

@alexander-beedie - sorry for the ping, just checking in on this.