[YannByron]

Purpose

The Python deferred BLOB reader bypasses FileIO range coalescing when blob_parallelism is 1, including the default. It reads each descriptor separately, even when descriptors point to adjacent or overlapping ranges in the same file.

Use the existing read_blobs_concurrent path for one worker as well. This lets serial reads merge ranges and reuse a stream while preserving result order, duplicate descriptors, nulls, table-scoped FileIO, and configured merge limits. For the regression fixture, the default path changes four individual payload reads into one merged range using one stream.

This change is limited to the Python deferred scalar BLOB reader; the Rust reader and the configured parallelism are unchanged.

Tests

• PYPAIMON_TEST_EXECUTION=python python -m pytest -q pypaimon/tests/deferred_blob_resolve_test.py pypaimon/tests/blob_test.py: 205 passed (Python 3.12.14, PyArrow 19.0.1).
• The three new parameterized cases fail against the unmodified upstream implementation and pass with the change. They cover default merging, a zero-gap limit, a maximum merged-span limit, duplicate/out-of-order descriptors, nulls, and closing the reused stream.
• Flake8 with the repository configuration passed for all three changed files.