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Ref #712, fix #622, fix #742

Before this change, any seek operation would close the current HTTP connection and open a new GET request to S3, even when seeking forward by a small amount within already-buffered data.

This commit adds an optimization to Reader.seek() that checks if a forward seek can be satisfied from the existing buffer. If the target position falls within the currently buffered range, the implementation simply advances the buffer position without making a new S3 request.

Backward seeks and forward seeks beyond the buffer still require new GET requests as before.

Tests

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1. Create a unit test that demonstrates the bug. The test should fail.
2. Implement your bug fix.
3. The test you created should now pass.

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pytest tests

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Checklist

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• Picked a concise, informative and complete title
• Clearly explained the motivation behind the PR
• Linked to any existing issues that your PR will be solving
• Included tests for any new functionality
• Run python update_helptext.py in case there are API changes
• Checked that all unit tests pass

Workflow

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