Casting pl.Categorical("physical") to pl.Categorical("lexical") doesn't invalidate sortedness flag

[evgenii-kuznetcov]

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

    df = pl.DataFrame({"s" : ["b", "a"], "v": [1, 2]})
    sorted_physically = df.cast({"s" : pl.Categorical("physical")}).sort("s")
    sorted_lexically = sorted_physically.cast({"s": pl.Categorical("lexical")}).sort("s")
    print(sorted_lexically)

Log output

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

Issue description

    df = pl.DataFrame({"s" : ["b", "a"], "v": [1, 2]})
    sorted_physically = df.cast({"s" : pl.Categorical("physical")}).sort("s")
    sorted_lexically = sorted_physically.cast({"s": pl.Categorical("lexical")}).sort("s")
    print(sorted_lexically)

In this example data should be sorted s: [a, b] but it is sorted s: [b, a].
Seems that polars remembers that sorted_physically is already sorted and doesn't sort it after it has been casted.
The issue has been introduced sometime between 1.18 and 1.19

Expected behavior

Data should be sorted s: [a, b] but it is sorted s: [b, a].

Installed versions

Polars:              1.20.0
Index type:          UInt32
Platform:            macOS-15.2-arm64-arm-64bit
Python:              3.10.12 (main, Oct 31 2023, 16:05:04) [Clang 15.0.0 (clang-1500.0.40.1)]
LTS CPU:             False

----Optional dependencies----
Azure CLI            <not installed>
adbc_driver_manager  <not installed>
altair               <not installed>
azure.identity       <not installed>
boto3                1.35.5
cloudpickle          3.0.0
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
google.auth          2.32.0
great_tables         <not installed>
matplotlib           3.9.2
nest_asyncio         <not installed>
numpy                2.1.0
openpyxl             <not installed>
pandas               <not installed>
pyarrow              <not installed>
pydantic             1.10.18
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>
None