full join with coalesce=True panics if more key expressions are used than columns in a frame

[wence-]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl
left = pl.DataFrame({"a": [1, 2, 3], "b": [3, 4, 5], "c": [5, 6, 7]})
right = pl.DataFrame({"a": [2, 3, 4], "c": [4, 5, 6]})

left.join(right, on=[pl.col("a"), pl.col("a") % 2 == 0, pl.col("a") + pl.col("c")], how="full", coalesce=True)
thread '<unnamed>' panicked at crates/polars-ops/src/frame/join/general.rs:90:25:
removal index (is 3) should be < len (is 3)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "/home/coder/doodles/python/polars-interp/bug2.py", line 5, in <module>
    left.join(right, on=[pl.col("a"), pl.col("a") % 2 == 0, pl.col("a") + pl.col("c")], how="full", coalesce=True)
  File "/home/coder/third-party/polars/py-polars/polars/dataframe/frame.py", line 6549, in join
    self.lazy()
  File "/home/coder/third-party/polars/py-polars/polars/lazyframe/frame.py", line 1855, in collect
    return wrap_df(ldf.collect(callback))
pyo3_runtime.PanicException: removal index (is 3) should be < len (is 3)

Log output

run JoinExec
join parallel: true

Issue description

This is followup to #16289, that case (repeated identical column expressions in the keys) was fixed in #16329, but this slightly more general case still panics

Expected behavior

No panic, and instead an appropriate error message.

FWIW, I think the case where coalesce=False is provided should also error, since it is (presumably) implementation defined which of the multiple key expressions becomes the concrete key value in the result (since they all have overlapping names).

Installed versions

--------Version info---------
Polars:               0.20.30
Index type:           UInt32
Platform:             Linux-6.5.0-35-generic-x86_64-with-glibc2.35
Python:               3.10.14 | packaged by conda-forge | (main, Mar 20 2024, 12:45:18) [GCC 12.3.0]

----Optional dependencies----
adbc_driver_manager:  1.0.0
cloudpickle:          3.0.0
connectorx:           0.3.3
deltalake:            0.17.4
fastexcel:            0.10.4
fsspec:               2024.5.0
gevent:               24.2.1
hvplot:               0.10.0
matplotlib:           3.9.0
nest_asyncio:         1.6.0
numpy:                1.26.4
openpyxl:             3.1.2
pandas:               2.2.2
pyarrow:              16.1.0
pydantic:             2.7.1
pyiceberg:            <not installed>
pyxlsb:               1.0.10
sqlalchemy:           2.0.30
torch:                2.3.0.post301
xlsx2csv:             0.8.2
xlsxwriter:           3.2.0