@pa.parser does not work for Index fields

[Girmii]

Describe the bug
A clear and concise description of what the bug is.

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• I have confirmed this bug exists on the latest version of pandera.
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Parser functions do not get called for fields typed as Index, although the documentation states: "Defines DataFrameModel parse methods for columns/indexes.".

Incidentally, I would not expect the parser for field B to be called twice, although the end result is correct.

Code Sample, a copy-pastable example

import pandas as pd
import pandera as pa
from pandera.typing import Index, Series


class TestModel(pa.DataFrameModel):
    A: Index[int]
    B: Series[int]
    
    class Config:
        coerce = True

    @pa.parser("A")
    def double(cls, series: pd.Series) -> pd.Series:
        print("parsing A")
        return series * 2

    @pa.parser("B")
    def triple(cls, series: pd.Series) -> pd.Series:
        print("parsing B")
        return series * 3


df = pd.DataFrame({"A": [1, 2, 3], "B": [1, 2, 3]})
df = DataFrame[TestModel](df)
print(df)

df = pd.DataFrame({"A": [1, 2, 3], "B": [1, 2, 3]})
df = df.set_index("A")  # Does not change the result
df = DataFrame[TestModel](df)
print(df)

Output

parsing B
parsing B

   A  B
0  1  3
1  2  6
2  3  9


parsing B
parsing B

   B
A   
1  3
2  6
3  9

Expected behavior

I would expect the parser function for 'A' to be applied to its respective Series.

Versions:

• Python 3.11.0
• pandas==2.2.0
• pandas-stubs==2.1.4.231227
• pandera==0.19.3
• mypy 1.10.0 (compiled: yes)