[python] Support BlobView fields

[leaves12138]

Summary

• Sync Python BLOB support with the BlobView work reviewed in [core] Support BlobView for shared blob data #7602, [codex] Support catalog-aware blob view function #7786, and [core] Allow blob inline fields without blob-field #7827.
• Add Python BlobViewStruct / BlobView wire-format support and the blob-view-field option.
• Store descriptor/view BLOB fields inline, validate bad inline field configuration and payloads, and avoid writing new .blob files for view fields.
• Resolve blob-view fields during reads through catalog-aware lookup, returning bytes by default or upstream BlobDescriptor bytes when blob-as-descriptor=true.

Codex

This PR was prepared and submitted by Codex.

Validation

• PYTHONPATH=paimon-python pytest -q paimon-python/pypaimon/tests/blob_test.py::BlobTest::test_blob_view_struct_roundtrip
• PYTHONPATH=paimon-python pytest -q paimon-python/pypaimon/tests/blob_table_test.py::DataBlobWriterTest::test_blob_view_fields_resolve_upstream_blob paimon-python/pypaimon/tests/blob_table_test.py::DataBlobWriterTest::test_blob_view_fields_rejects_non_view_input paimon-python/pypaimon/tests/blob_table_test.py::DataBlobWriterTest::test_blob_inline_fields_reject_overlap_and_unknown_fields paimon-python/pypaimon/tests/blob_table_test.py::DataBlobWriterTest::test_blob_descriptor_fields_mixed_mode paimon-python/pypaimon/tests/blob_table_test.py::DataBlobWriterTest::test_to_arrow_batch_reader paimon-python/pypaimon/tests/blob_table_test.py::DataBlobWriterTest::test_blob_descriptor_fields_rejects_non_descriptor_input
• PYTHONPATH=paimon-python python3 -m flake8 --config=paimon-python/dev/cfg.ini paimon-python/pypaimon/table/row/blob.py paimon-python/pypaimon/common/options/core_options.py paimon-python/pypaimon/schema/schema.py paimon-python/pypaimon/write/writer/data_blob_writer.py paimon-python/pypaimon/read/reader/data_file_batch_reader.py paimon-python/pypaimon/read/reader/blob_descriptor_convert_reader.py paimon-python/pypaimon/utils/blob_view_lookup.py paimon-python/pypaimon/tests/blob_test.py paimon-python/pypaimon/tests/blob_table_test.py
• python3 -m compileall -q paimon-python/pypaimon/table/row/blob.py paimon-python/pypaimon/common/options/core_options.py paimon-python/pypaimon/schema/schema.py paimon-python/pypaimon/write/writer/data_blob_writer.py paimon-python/pypaimon/read/reader/data_file_batch_reader.py paimon-python/pypaimon/read/reader/blob_descriptor_convert_reader.py paimon-python/pypaimon/utils/blob_view_lookup.py
• git diff --check

[leaves12138]

Thanks for the PR. The overall direction looks right to me: the Python BlobViewStruct wire format matches the Java side, and the write/read path for inline blob-view-field plus blob-as-descriptor=true is mostly aligned with the BlobView design.

I found one blocker around the data-evolution read path though.

DataFileBatchReader already resolves inline blob-view-field values to the actual blob payload when blob-as-descriptor=false. In the data-evolution path, DataEvolutionSplitRead then wraps the merged reader with BlobDescriptorConvertReader, and that reader scans the already-resolved bytes again with BlobViewStruct.is_blob_view_struct() / deserialize(). This means a perfectly valid blob payload whose first bytes happen to match the BlobView magic header can be parsed as a BlobViewStruct and fail, or even be incorrectly resolved as another view.

The problematic chain is:

• DataFileBatchReader._convert_inline_blob_columns resolves view fields before returning file batches.
• DataEvolutionSplitRead.create_reader wraps the result with BlobDescriptorConvertReader whenever blob descriptor/view fields exist.
• BlobDescriptorConvertReader._convert_batch attempts to interpret the final bytes as BlobViewStruct again.

I reproduced this locally by writing a source blob payload starting with the BlobView version + magic bytes and then reading it through a downstream blob-view-field; the default read failed with ValueError: Invalid BlobViewStruct data: too short even though the source payload is just normal blob data.

I think we should ensure inline blob values are converted exactly once. One possible fix is to keep conversion in DataFileBatchReader and not wrap data-evolution readers with BlobDescriptorConvertReader for fields that have already been converted; alternatively, disable the file-reader-side conversion in the data-evolution path and do conversion only after merge/filter.

One more compatibility point: Java validation requires fields listed in blob-descriptor-field / blob-view-field to also be listed in blob-field. The Python schema validation currently only checks that those names are BLOB fields and do not overlap. If this relaxation is intentional because Python treats all large_binary columns as BLOBs, please document it; otherwise it would be better to align the option validation with Java to avoid creating schemas that Java/Flink would reject.

Local validation I ran:

• python3.8 -m pytest -q paimon-python/pypaimon/tests/blob_table_test.py paimon-python/pypaimon/tests/blob_test.py -> 87 passed, 1 skipped
• targeted BlobView tests -> passed
• flake8 on changed Python files -> passed
• compileall on changed Python files -> passed
• git diff --check -> passed