[python] Avoid redundant blob stream opens on index cache hits

[XiaoHongbo-Hope]

Purpose

Avoid opening a BLOB file when its cached index is sufficient to return scalar descriptors. Open streams on index misses and when payloads, nested layouts, or video metadata require bytes. Keep the cache, public API, and file format unchanged.

Validation

• Real local files with known sizes: cold + warm descriptor reads reduce opens from 2 to 1, with identical results.
• 259 related tests passed, covering payload reads, ARRAY/MAP BLOB, row selection, reader isolation, error cleanup, deferred resolution, and video.
• flake8, license checks, and git diff --check passed.

Measurements count Python FileIO calls; OSS HEAD/GET traffic and Rust native reads were not measured.