fix: Ensure mean_horizontal raises on non-numeric input

[alexander-beedie]

The mean_horizontal expression was the only specialised horizontal function that wouldn't raise an error on incompatible input - instead it just filtered that input for the numeric/boolean columns, checked there was at least one, and operated on those.

This is inconsistent; selectors can be used to trivially choose numeric columns - the expression shouldn't be doing it silently (and it isn't documented that it would do so).

Incompatible input now raises a clear error.

Also:

Allow Decimal in mean_horizontal (was omitted by an .is_numeric() dtype check).

Example

• Catch invalid input

  from decimal import Decimal as D
  import polars as pl
  
  df = pl.DataFrame({
      "cola": [D("1.5"), D("0.5"), D("5"), D("0"), D("-0.25")],
      "colb": [[0,1], [2], [3,4], [5], [6]],
      "colc": ["aa", "bb", "cc", "dd", "ee"],
      "cold": ["bb", "cc", "dd", "ee", "ff"],
      "cole": [1000, 2000, 3000, 4000, 5000],
  })
  df.select(
      pl.mean_horizontal(
          pl.col("cola"),
          pl.col("colb"),
          pl.col("colc"),
          pl.col("cold"),
          pl.col("cole"),
      )
  )

  Before:

  # shape: (5, 1)
  # ┌────────┐
  # │ cola   │
  # │ ---    │
  # │ f64    │
  # ╞════════╡
  # │ 1000.0 │
  # │ 2000.0 │
  # │ 3000.0 │
  # │ 4000.0 │
  # │ 5000.0 │
  # └────────┘

  After:

  # InvalidOperationError: 
  # 'horizontal_mean' expects numeric expressions, found "colb" (dtype=list[i64])

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.92%. Comparing base (047e578) to head (88e250d).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19648   +/-   ##
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  Files        1536     1536           
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  Branches     2445     2445           
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  Misses      41946    41946           
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[ritchie46]

Good fix. Just a mental note to us, that we ideally check this in the IR resolve phase.

[alexander-beedie]

Good fix. Just a mental note to us, that we ideally check this in the IR resolve phase.

We should likely allow temporal expressions here too (then validate the supertype of all inputs), but one thing at a time :)