.Net: VectorStore: Add unit tests for mappers and adding dedicated options for mapper.

[westey-m]

Motivation and Context

As part of adding new vector stores for redis and qdrant, these include mappers from data models to storage models and back, that require unit testing.
The mapper interface also uses an options parameter which was reused from the IVectorRecordStore.GetAsync method, but really should have been a dedicated options object from the start.

Description

• Added unit tests for the redis and qdrant data model mappers.
• Added a dedicated options model for the mapper.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄