### What problem does this PR solve?
- Add aggregation_utils.aggregate_by_field for pure aggregation logic
- Wire OBConnection.get_aggregation to use it (unwrap tuple, pass
messages)
- Add unit tests for aggregate_by_field (no DB/heavy deps)
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Add OceanBase memory store and extracting base class `OBConnectionBase`.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
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Co-authored-by: Cursor <cursoragent@cursor.com>
