[SPARK-56254][PYTHON][CONNECT] Make spark.read.xml accept DataFrame input

[Yicong-Huang]

What changes were proposed in this pull request?

This PR adds DataFrame input support to spark.read.xml(), following the same pattern established by spark.read.json() (SPARK-56253) and spark.read.csv() (SPARK-56255).

Why are the changes needed?

This completes the DataFrame input support across all text-based readers (JSON, CSV, XML).

Does this PR introduce any user-facing change?

Yes. spark.read.xml() now accepts a DataFrame with a single string column as input, in addition to the existing path/list/RDD inputs.

Users can now parse XML strings stored in a DataFrame column directly:

xml_df = spark.createDataFrame(
    [('<person><name>Alice</name><age>25</age></person>',),
     ('<person><name>Bob</name><age>30</age></person>',)],
    schema="value STRING",
)
spark.read.option("rowTag", "person").xml(xml_df).show()

How was this patch tested?

Added 10 new tests (5 classic + 5 Connect)

Was this patch authored or co-authored using generative AI tooling?

No