[SPARK-43847][PYTHON] Throw structured error when reading Protobuf descriptor file fails

[brijrajk]

What changes were proposed in this pull request?

_read_descriptor_set_file() in pyspark.sql.protobuf.functions now raises
PySparkRuntimeError with error class PROTOBUF_DESCRIPTOR_FILE_NOT_FOUND
instead of propagating a raw Python OSError / FileNotFoundError when the
descriptor file cannot be read.

The error class and its message already existed on the Scala side
(common/utils/src/main/resources/error/error-conditions.json); this PR adds
the matching entry to python/pyspark/errors/error-conditions.json and wires
it up in the Python code.

Why are the changes needed?

Before this fix, users who pass a wrong or missing descriptor file path get a
bare Python OS error with no Spark context:

FileNotFoundError: [Errno 2] No such file or directory: '/path/to/file.desc'

After this fix they get a structured Spark error consistent with the rest of
the PySpark error framework:

PySparkRuntimeError: [PROTOBUF_DESCRIPTOR_FILE_NOT_FOUND]
Error reading Protobuf descriptor file at path: /path/to/file.desc.

Does this PR introduce any user-facing change?

Yes. Users who catch OSError / FileNotFoundError around from_protobuf /
to_protobuf calls that supply a descFilePath will now need to catch
PySparkRuntimeError instead (or the common base PySparkException). The
error message is clearer and includes the file path.

How was this patch tested?

Added python/pyspark/sql/tests/test_protobuf.py with
test_read_descriptor_set_file_not_found, which asserts that a missing path
raises PySparkRuntimeError with the correct error class and filePath
message parameter.

Run locally with:

PYTHONPATH=python python3 -m unittest pyspark.sql.tests.test_protobuf -v

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

Generated-by: Claude Sonnet 4.6

[brijrajk]

Could a committer please review this? It raises a structured PySparkRuntimeError with error class PROTOBUF_DESCRIPTOR_FILE_NOT_FOUND instead of a bare FileNotFoundError when the descriptor file path is invalid — consistent with the rest of the PySpark error framework (SPARK-43847).

cc @HyukjinKwon @LuciferYang