.Net: Inconsistent results when creating a collection that already exists

[markwallace-microsoft]

Different storage implementations handle this case differently:

• Most return OK, without checking if the existing collection matches the required schema.
• InMemoryVectorStore throws an exception.

Suggestion:

• Provide an option to fail explicitly if a collection already exists, so the client can handle the scenario and doesn’t risk using incorrect schemas
• Allow schema validation when reusing an existing collection. This might be hard, the previous point would mitigate the problem.

[roji]

Thanks @dluc. I don't think we should provide an option here - we already have both CreateCollection and CreateCollectionIfExists, IMHO the former should just fail if the collection exists, just as the creation generally fails in the actual low-level databases.

/cc @adamsitnik this is another good scenario for enforcing via the conformance tests.

[dluc]

Agreed, this was likely a mistake on my part during the analysis, ok to close.

[adamsitnik]

I am reopening the issue as we need to make sure the behavior is following:

• CreateCollection does throw VectorStoreOperationException when called more than once
• CreateCollectionIfNotExistsAsync does not.

I've already provided a PR with the conformance tests in #10944, will send a separate one with unification.