[python] Route descriptor-backed BLOB reads through the table FileIO.

[Stephen0421]

Purpose

Route descriptor-backed BLOB reads through the table FileIO, and resolve known descriptor columns with from_descriptor_bytes instead of the v2-only from_bytes() heuristic.

• UriReaderFactory.from_file_io matches Java fromFileIO: HTTP(S) stays on HttpUriReader; every other URI reuses the table FileIO (REST tokens). Non-HTTP readers are not LRU-cached, so the factory does not pin FileIO. After pickle, a ResolvingFileIO table still works once uri_reader_factory has been materialized.
• Convert reader and OffsetRow.get_blob() parse descriptor-field bytes with from_descriptor_bytes (v1, trailing padding, and VideoFrameDescriptor via serde).
• After materializing payload (blob-as-descriptor=false), descriptor field indices are cleared so row-level get_blob() does not re-parse payload as a descriptor. Reader wrappers refresh blob-view lookup only and do not overwrite those indices.
• Apply LIMIT before inline convert on merge splits, keep RowKind through the row/batch/row adapter, and keep sequence.field on blob-view prescan so overlapping-file merge can still compare.
• read_blobs_concurrent coalesces exact BlobRef + FileUriReader only; subclasses keep new_input_stream().

Out of scope: when blob-as-descriptor=true, Stage 1 still rewrites BlobViewStruct to serialized descriptor bytes. Keeping the struct for that mode is a follow-up.

Tests

• BlobTest: v1/trailing-padding/VideoFrameDescriptor via from_descriptor_bytes; table FileIO on convert/get_blob/deferred resolve; LIMIT before convert; RowKind through adapters; exact-BlobRef coalesce; merge blob-view prescan keeps sequence.field; _refresh_blob_view_lookup does not clobber descriptor indices
• DedicatedFormatWriterTest.test_blob_view_predicate_and_limit_resolves_filtered_row
• DedicatedFormatWriterTest.test_blob_view_raw_split_predicate_and_limit_resolves_filtered_row
• UriReaderFactoryTest: from_file_io reuses FileIO for non-HTTP, does not pin FileIO, and pickles with FileIO in the graph
• ResolvingFileIOTest: pickle FileIO and table after uri_reader_factory is materialized
• RESTTokenFileIOTest.test_close_only_closes_instance_uri_reader_factory