fix: Error instead of truncate if length mismatch for several str functions

[etiennebacher]

Fixes #16070

The original issue only mentioned str.zfill(). There was already a similar check for some functions (e.g. str.head()). This PR extends this check to more str functions and adds tests, but I did not check for other namespaces.

Before (1.20.0): silently truncate to parameter length

import polars as pl

input = pl.Series(["foo", "bar", "baz"])

input.str.starts_with(pl.Series(["a", "b"]))
input.str.ends_with(pl.Series(["a", "b"]))
input.str.contains(pl.Series(["a", "b"]))
input.str.find(pl.Series(["a", "b"]))
input.str.zfill(pl.Series([1, 2]))
input.str.strip_chars(pl.Series(["a", "b"]))
input.str.strip_chars_start(pl.Series(["a", "b"]))
input.str.strip_chars_end(pl.Series(["a", "b"]))
input.str.strip_prefix(pl.Series(["a", "b"]))
input.str.strip_suffix(pl.Series(["a", "b"]))

shape: (2,)
Series: '' [bool]
[
        false
        true
]
shape: (2,)
Series: '' [bool]
[
        false
        false
]
shape: (2,)
Series: '' [bool]
[
        false
        true
]
shape: (2,)
Series: '' [u32]
[
        null
        0
]
shape: (2,)
Series: '' [str]
[
        "foo"
        "bar"
]
shape: (2,)
Series: '' [str]
[
        "foo"
        "ar"
]
shape: (2,)
Series: '' [str]
[
        "foo"
        "ar"
]
shape: (2,)
Series: '' [str]
[
        "foo"
        "bar"
]
shape: (2,)
Series: '' [str]
[
        "foo"
        "ar"
]
shape: (2,)
Series: '' [str]
[
        "foo"
        "bar"
]

Now: error

polars.exceptions.ComputeError: all series in `str.starts_with()` should have equal or unit length

... etc() ...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.78%. Comparing base (28a45f2) to head (8fbc4ab).
Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20781      +/-   ##
==========================================
+ Coverage   79.73%   79.78%   +0.04%     
==========================================
  Files        1561     1561              
  Lines      221910   221994      +84     
  Branches     2530     2531       +1     
==========================================
+ Hits       176938   177115     +177     
+ Misses      44390    44297      -93     
  Partials      582      582              

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[eitsupi]

Was this change really necessary?
This change broke the R API by preventing the use of str type Series of longer than length 2 as input.