[python] Fix DataBlobWriter KeyError for partial writes with blob columns

[SteNicholas]

Purpose

Problem: For tables with BLOB columns, pypaimon uses DataBlobWriter, which splits each pyarrow.RecordBatch into “normal” columns (written to Parquet/ORC/…) and blob-file columns (written via BlobWriter). _split_data used the full table lists of normal and blob-file column names when calling RecordBatch.select(...).

Regression: When TableWrite.with_write_type(...) narrows the write to a partial column list, validation ensures incoming batches only contain those columns. _split_data still tried to select columns not present in the batch (e.g. a normal column omitted from the partial write), which caused PyArrow to raise KeyError.

Fix:

• Pass write_cols from FileStoreWrite into DataBlobWriter (same as for AppendOnlyDataWriter), so the blob writer sees the narrowed column set from with_write_type.
• In DataBlobWriter.__init__, derive normal_column_names and blob_file_column_names from that subset when write_cols is set: only blob-file columns that appear in write_cols, and normal columns = write_cols minus blob-file columns (order preserved from write_cols). Only instantiate BlobWriter for blob-file columns in that narrowed set.
• Keep full-table behavior when write_cols is None (full schema write).
• This keeps _split_data consistent with the actual batch schema and matches the intent of partial / data-evolution writes.

Close #7849.

Tests

Added in paimon-python/pypaimon/tests/blob_table_test.py(DataBlobWriterTest):

1. Partial normal + one blob — with_write_type(['id', 'blob_data']) with a batch that only has those columns; asserts one Parquet file (write_cols == ['id']), blob file(s) (write_cols == ['blob_data']), row counts, commit + read-back (unwritten name is null).
2. Partial normal only — with_write_type(['id', 'name']) without blob columns in the batch; asserts no .blob files, Parquet write_cols == ['id', 'name'], read-back with blob column null.
3. Two blob columns, write one — schema with blob1 and blob2, with_write_type(['id', 'blob1']); asserts exactly one blob file and write_cols == ['blob1'].

[JingsongLi]

+1