Incorrect sorted flag propagation when casting from numeric to string type

[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

df = pl.DataFrame({"a": [10, 1, 2]})

df.select(
  pl.col("a").sort().cast(pl.String).sort().alias("sort_cast_sort"),
  pl.col("a").cast(pl.String()).sort().alias("cast_sort")
)

Produces:

shape: (3, 2)
┌────────────────┬───────────┐
│ sort_cast_sort ┆ cast_sort │
│ ---            ┆ ---       │
│ str            ┆ str       │
╞════════════════╪═══════════╡
│ 1              ┆ 1         │
│ 2              ┆ 10        │
│ 10             ┆ 2         │
└────────────────┴───────────┘

And indeed:

df.select(pl.col("a").sort().cast(pl.String)).flags
# {'a': {'SORTED_ASC': True, 'SORTED_DESC': False}}

Log output

No response

Issue description

Casting from numeric to string types shouldn't propagate sortedness metadata, since string types sort lexicographically, not numerically.

Expected behavior

sort-cast-sort should produce the same result as cast-sort.

Installed versions

--------Version info---------
Polars:              1.11.0
Index type:          UInt32
Platform:            Linux-6.8.0-47-generic-x86_64-with-glibc2.35
Python:              3.12.7 | packaged by conda-forge | (main, Oct  4 2024, 16:05:46) [GCC 13.3.0]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          3.1.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2024.10.0
gevent               <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         1.6.0
numpy                2.0.2
openpyxl             3.1.5
pandas               2.2.3
pyarrow              17.0.0
pydantic             2.9.2
pyiceberg            <not installed>
sqlalchemy           2.0.36
torch                2.4.1.post302
xlsx2csv             <not installed>
xlsxwriter           <not installed>