fix: Mark Expr.(lower|upper)_bound as returning scalar

[orlp]

Necessary for the new streaming engine, but also fixes this (obscure) bug in the current engine:

>>> df = pl.DataFrame({"a": [1, 2, 3, 2, 1]})
>>> df.group_by("a").agg(pl.col("a").lower_bound().alias("lower"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/orlp/.localpython/lib/python3.11/site-packages/polars/dataframe/group_by.py", line 231, in agg
    .collect(no_optimization=True)
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/orlp/.localpython/lib/python3.11/site-packages/polars/lazyframe/frame.py", line 2027, in collect
    return wrap_df(ldf.collect(callback))
                   ^^^^^^^^^^^^^^^^^^^^^
polars.exceptions.InvalidOperationError: output length of `map` (1) must be equal to the input length (5); consider using `apply` instead

Error originated in expression: 'col("a").lower_bound()'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.82%. Comparing base (f07eea3) to head (fef2c78).
Report is 67 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18383   +/-   ##
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  Coverage   79.81%   79.82%           
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  Files        1496     1496           
  Lines      200338   200346    +8     
  Branches     2841     2841           
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+ Hits       159902   159921   +19     
+ Misses      39911    39900   -11     
  Partials      525      525           

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