Implement score threshold for Azure AI Search (when out of preview)

In [.Net: VectorStore: Add ability to filter results by the similarity score (microsoft/semantic-kernel#9566)](https://github.com/microsoft/semantic-kernel/issues/9566), we added MEVD support for score thresholds, to avoid fetching low-quality results. Azure AI Search does support this, but [in preview only at this point](https://learn.microsoft.com/azure/search/vector-search-how-to-query?tabs=query-2025-09-01%2Cbuiltin-portal#set-thresholds-to-exclude-low-scoring-results-preview).

For now, we've implemented client-side post-filtering. Once the feature is no longer in preview we should use it.