[SPARK-56255][PYTHON][CONNECT] Make spark.read.csv accept DataFrame input

[Yicong-Huang]

What changes were proposed in this pull request?

This PR adds support for passing a DataFrame containing CSV strings directly to spark.read.csv(), following the same pattern established by #55097 (SPARK-56253) for spark.read.json().

Changes:

• Classic PySpark (readwriter.py): Updated csv() to accept DataFrame input, delegating to PythonSQLUtils.csvFromDataFrame().
• Spark Connect (connect/readwriter.py): Updated csv() to accept DataFrame input, using the existing Parse logical plan with PARSE_FORMAT_CSV.
• Scala bridge (PythonSQLUtils.scala): Added csvFromDataFrame() method that validates the input DataFrame has a StringType first column, then delegates to DataFrameReader.csv().
• Tests: Added 5 test cases each for classic and Connect (basic input, schema override, non-string column error, multiple columns, zero columns).

Why are the changes needed?

spark.read.json() already supports DataFrame input (SPARK-56253), but spark.read.csv() does not. This inconsistency means users who want to parse CSV strings stored in a DataFrame must use workarounds. Adding DataFrame support to csv() makes the API consistent with json() and enables Connect-compatible CSV parsing without sc.parallelize().

Does this PR introduce any user-facing change?

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

csv_df = spark.createDataFrame([("Alice,25",), ("Bob,30",)], schema="value STRING")
spark.read.csv(csv_df, schema="name STRING, age INT").show()
# +-----+---+
# | name|age|
# +-----+---+
# |Alice| 25|
# |  Bob| 30|
# +-----+---+

How was this patch tested?

Added 10 new test cases:

• test_csv_with_dataframe_input (classic + connect)
• test_csv_with_dataframe_input_and_schema (classic + connect)
• test_csv_with_dataframe_input_non_string_column (classic + connect)
• test_csv_with_dataframe_input_multiple_columns (classic + connect)
• test_csv_with_dataframe_input_zero_columns (classic + connect)

All tests pass locally.

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

No.