fix: implement _exists for SharedMemoryDataset

[felixscherz]

Description

This adds an _exists method to the SharedMemoryDataset that calls the underlying MemoryDataset instead of relying on the inherited AbstractDataset._exists method.
This is intended to fix #3703

Development notes

Implemented _exists for SharedMemoryDataset and added a test to verify that SharedMemoryDataset.exists accurately reflects the state of the underlying MemoryDataset.

Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team