[python] Keep BlobViewStruct bytes when blob-as-descriptor is true.

[Stephen0421]

Purpose

When blob-as-descriptor=true, keep BlobViewStruct bytes through Stage 1 so get_blob() still resolves through the originating table FileIO. Serialize descriptors only at the Arrow/pandas output boundary.

Previously Stage 1 replaced view bytes with a plain descriptor. That dropped the upstream-table association, so get_blob() used the target FileIO to open the URI. Under REST catalog that is the wrong token (403).

• Convert reader still prescans and builds BlobViewLookup, but does not rewrite view columns when blob-as-descriptor=true.
• TableRead converts view fields to descriptor bytes in to_arrow / to_pandas / to_arrow_batch_reader (serial and parallel). to_iterator() is unchanged.
• Cache blob-as-descriptor and _blob_view_output_indices (positions in read_type, not the widened _scan_read_type) so non-view row paths skip per-row option lookup.

Follow-up to #9608. Native read (read.native.enabled) is unchanged and still defaults to false; view tables should stay on the Python reader until native grows a matching hook.

Tests

• test_blob_view_as_descriptor_get_blob_uses_upstream_file_io: guarded FileIO; blob-as-descriptor=true get_blob().to_data() must not read source .blob with the target FileIO.
• test_blob_view_as_descriptor_projection_with_predicate_extra_field: projection ['picture', 'id'] plus a predicate on unprojected grp; _blob_view_output_indices == (0,), Arrow has only those two columns, and picture is descriptor bytes.
• test_blob_view_fields_resolve_upstream_blob: to_arrow still returns BlobDescriptor bytes when blob-as-descriptor=true.
• test_blob_view_default_read_uses_upstream_file_io: default (blob-as-descriptor=false) still materializes payload with the upstream FileIO.
• test_table_read_serializes_only_configured_blob_view_fields: only configured view columns are rewritten; empty indices / blob-as-descriptor=false skip serialize.
• test_offset_row_get_blob_view_keeps_per_table_uri_reader: get_blob() keeps the originating table UriReader.
• test_offset_row_get_blob_v1_resolved_blob_view_field / test_offset_row_get_blob_resolves_null_blob_view / test_offset_row_get_blob_view_struct_without_view_field_indices: view-struct get_blob() for v1, null, and missing view-field indices.
• test_to_iterator_adapters_refresh_blob_view_lookup_after_first_read: merge to_iterator adapters copy lookup after the first convert read.

[JingsongLi]

Requirement fit: SUPPORTED. A BlobView must retain its source-table FileIO when get_blob() resolves it; eagerly replacing it with descriptor bytes under blob-as-descriptor mode loses that association. The patch defers descriptor serialization to Arrow output and exercises row, Arrow, adapter, and source-IO paths. Implementation: CLEAN in the reviewed production diff. I did not rerun the Python suite locally.