Port dotnet feature: .Net: Add delete and exists methods for IVectorStore

[github-actions]

Original issue

#11596

Description

Motivation and Context

#10881

Description

• Add CollectionExists and DeleteCollection methods to VectorStore
• Add implementations for all vector stores that proxy to the collection
• Add integration tests
• Make some small fixes for some integration tests.

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 😄

\n Relates to #11596