fix: Fix BlobReader handling of interleaved reads and seeks

[aabbccddeeffgghhii1438]

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Fixes #462 🦕

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[andrewsg]

Thank you for your valuable contribution and especially for adding the test. I can see the problem was due to an inconsistent usage of reader._pos between the read and seek methods. I have one suggestion, which will fix the DeprecationWarnings in pytest output.

[andrewsg]

Thank you! Will merge once system tests are complete.