fix: Right join on multiple columns not coalescing left_on columns

[lukemanley]

fixes #20670
fixes #20671

Right join on multiple columns where left_on and right_on names do not match was failing to coalesce the left_on columns. The lazy schema was already producing the correct behavior.

import polars as pl

left = pl.LazyFrame({"a": [1, 2], "b": [1, 2]})
right = pl.LazyFrame({"c": [1, 2], "d": [1, 2]})

print(
    left.join(right, left_on=["a", "b"], right_on=["c", "d"], how="left").collect()
)

# shape: (2, 2)
# ┌─────┬─────┐
# │ a   ┆ b   │
# │ --- ┆ --- │
# │ i64 ┆ i64 │
# ╞═════╪═════╡
# │ 1   ┆ 1   │
# │ 2   ┆ 2   │
# └─────┴─────┘

print(
    left.join(right, left_on=["a", "b"], right_on=["c", "d"], how="right").collect()
)

# BEFORE:

# shape: (2, 4)
# ┌─────┬─────┬─────┬─────┐
# │ a   ┆ b   ┆ c   ┆ d   │
# │ --- ┆ --- ┆ --- ┆ --- │
# │ i64 ┆ i64 ┆ i64 ┆ i64 │
# ╞═════╪═════╪═════╪═════╡
# │ 1   ┆ 1   ┆ 1   ┆ 1   │
# │ 2   ┆ 2   ┆ 2   ┆ 2   │
# └─────┴─────┴─────┴─────┘

# AFTER:

# shape: (2, 2)
# ┌─────┬─────┐
# │ c   ┆ d   │
# │ --- ┆ --- │
# │ i64 ┆ i64 │
# ╞═════╪═════╡
# │ 1   ┆ 1   │
# │ 2   ┆ 2   │
# └─────┴─────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.47%. Comparing base (bcdc7da) to head (cddca79).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21669      +/-   ##
==========================================
- Coverage   80.47%   80.47%   -0.01%     
==========================================
  Files        1605     1605              
  Lines      231842   231847       +5     
  Branches     2678     2678              
==========================================
+ Hits       186567   186569       +2     
- Misses      44647    44649       +2     
- Partials      628      629       +1     

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[ritchie46]

Nice!