fix(dataset): correct BlobFile seek semantics - #9358
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Python added a relative offset then passed it as u64. seek(-n, SEEK_CUR) at 0 raised OverflowError. BytesIO raises ValueError. Zip and mp4 readers catch ValueError. read() snapped a past-EOF cursor back to size. tell() stays where the caller seeked.
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✅ Gate recommendation: approve.
This restores file-like cursor semantics at the correct boundaries: negative final seek positions are rejected before unsigned conversion without moving the cursor, while reads beyond EOF return empty bytes without moving the cursor backward. The regression coverage exercises all seek origins and both Python and Rust behavior.
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Followup to #9330.
Seeking before the start of a blob raised
OverflowErrorwhen Python passed the negative position to Rust as au64. This now raisesValueErrorand leaves the cursor unchanged.Also fixes
read()moving a cursor past EOF back to EOF. It now returns empty bytes and leaves the cursor where it was.Testing
Added Python and Rust regression tests.