.Net: [MEVD] API review changes to collection creation

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From API review:

• Rename IVectorStore.CreateCollectionIfNotExistsAsync to EnsureCollectionExistsAsync.
• Remove CreateCollectionAsync; we're not aware of non-contrived mainstream scenarios where users would actually need an exception to be thrown if the collection already exists.
• Ensure that EnsureCollectionExistsAsync implementations are thread-safe.
  • This also covers calling EnsureCollectionExistsAsync from two separate .NET applications concurrently; in other words, an implementation that checks for existence and then calls a throwing CreateCollection isn't good, as that can fail.
  • Where a native idempotent database implementation doesn't exist, we can call a throwing CreateCollection and then recognize and ignore the "collection already exists" error.