fix(python): Selectors should raise on + between themselves

[alexander-beedie]

Closes #20821.

Selectors as sets

Selectors act as sets and support the standard set ops:
https://docs.pola.rs/api/python/stable/reference/selectors.html#set-operations

Operation	Expression
UNION	A | B
INTERSECTION	A & B
DIFFERENCE	A - B
SYMMETRIC DIFFERENCE	A ^ B
COMPLEMENT	~A

Standard set behaviour

However... sets do not support the + operator between themselves.

{1,2,3} + {2,3,4}
# TypeError: unsupported operand type(s) for +: 'set' and 'set'

We are inadvertently passing-through + between selectors to Expr, leading to very peculiar (and unintended) results or errors in cases where the caller meant to use | or & instead. This PR fixes that (while retaining support for broadcasting).

Example

import polars as pl
import polars.selectors as cs

data = {
    "col1": [1, 2, 3],
    "col2": [4.1, 5.2, 6.3],
    "col3": ["x", "y", "z"],
}
df = pl.DataFrame(data)

Use of + between selectors now raises...

df.select(cs.numeric() + cs.string())
# TypeError: unsupported operand type(s) for op: ('Selector' + 'Selector')

...but continues to broadcast:

df.select(cs.numeric() + 100)
# shape: (3, 2)
# ┌──────┬───────┐
# │ col1 ┆ col2  │
# │ ---  ┆ ---   │
# │ i64  ┆ f64   │
# ╞══════╪═══════╡
# │ 101  ┆ 104.1 │
# │ 102  ┆ 105.2 │
# │ 103  ┆ 106.3 │
# └──────┴───────┘

[MarcoGorelli]

nice one!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.69%. Comparing base (099ee3c) to head (bf228b0).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20825      +/-   ##
==========================================
+ Coverage   79.63%   79.69%   +0.06%     
==========================================
  Files        1568     1568              
  Lines      222970   222981      +11     
  Branches     2544     2545       +1     
==========================================
+ Hits       177555   177702     +147     
+ Misses      44831    44695     -136     
  Partials      584      584              

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