[SPARK-57192][PYSPARK] Fix classic addArtifacts with multiple paths

[wbo4958]

What changes were proposed in this pull request?

This PR fixes classic PySpark SparkSession.addArtifacts to handle multiple paths in
one call.

The classic implementation previously forwarded all paths as positional arguments to
SparkContext.addPyFile, SparkContext.addArchive, or SparkContext.addFile. Those
APIs accept one path at a time, so this PR updates classic addArtifacts to call the
underlying SparkContext API once per path.

This PR also adds regression coverage for adding multiple Python files in a single
addArtifacts(..., pyfile=True) call.

Why are the changes needed?

Classic PySpark currently fails for useful multi-path calls such as:

spark.addArtifacts("a.py", "b.py", "c.py", pyfile=True)

with:

TypeError: SparkContext.addPyFile() takes 2 positional arguments but 4 were given

The public API accepts *path, so classic Spark should support multiple artifacts
consistently.

Does this PR introduce any user-facing change?

Yes. Classic PySpark users can now add multiple artifacts in one
SparkSession.addArtifacts call when using pyfile=True, archive=True, or file=True.

How was this patch tested?

Added a regression test for adding multiple Python files in one call.

Also manually ran:

env -u SPARK_CONF_DIR PYTHONPATH="python:python/lib/py4j-0.10.9.9-src.zip"
SPARK_HOME="$PWD" \
    python3 -m unittest pyspark.sql.tests.test_artifact.ArtifactTests.test_add_multiple_pyfiles

and a standalone local Spark repro covering pyfile=True, file=True, and archive=True.

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

Generated-by: OpenAI Codex (GPT-5.5)

[wbo4958]

Hi @HyukjinKwon, @zhengruifeng, @cloud-fan, Could you help review this PR? Thx

[HyukjinKwon]

Merged to master and branch-4.x.