arrow

GH-38034: [Python] DataFrame Interchange Protocol - correct dtype information for categorical columns (#38065)

### Rationale for this change
See: https://github.com/apache/arrow/issues/38034#issue-1927839216

### What changes are included in this PR?

The `f_string` for the columns with categorical dtype is now corrected to reflect the type of the indices from the dictionary data type. Bit width has been correct before. From the spec:

> For categoricals, the format string describes the type of the
              categorical in the data buffer. In case of a separate encoding of
              the categorical (e.g. an integer to string mapping), this can
              be derived from ``self.describe_categorical``.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #38034

Authored-by: AlenkaF <frim.alenka@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>