Add streaming support and improve JSON serialization

- Updated `Response` record class to allow nullable `Question` and `Answer` properties; moved `StreamState` enum to a new file.
- Added `JsonStringEnumConverter` in `Program.cs` for better enum serialization.
- Corrected terminology in document upload endpoint description.
- Introduced `/api/ask-streaming` endpoint for streaming question responses.
- Added `AskStreamingAsync` method in `VectorSearchService` for handling streaming logic.
- Created `StreamState.cs` to define `StreamState` enum with `Start`, `Append`, and `End` values.

SqlDatabaseVectorSearch/Services/VectorSearchService.cs

@@ -88,6 +88,25 @@ public class VectorSearchService(ApplicationDbContext dbContext, ITextEmbeddingG
         return new Response(reformulatedQuestion, answer);
     }
 
+    public async IAsyncEnumerable<Response> AskStreamingAsync(Question question, bool reformulate = true)
+    {
+        var (reformulatedQuestion, chunks) = await CreateContextAsync(question, reformulate);
+
+        var answerStream = chatService.AskStreamingAsync(question.ConversationId, chunks, reformulatedQuestion);
+
+        // The first message contains the original question.
+        yield return new Response(reformulatedQuestion, null, StreamState.Start);
+
+        // Return each token as a partial response.
+        await foreach (var token in answerStream)
+        {
+            yield return new Response(null, token, StreamState.Append);
+        }
+
+        // The last message tells the client that the stream has ended.
+        yield return new Response(null, null, StreamState.End);
+    }
+
     private async Task<(string Question, IEnumerable<string> Chunks)> CreateContextAsync(Question question, bool reformulate = true)
     {
         // Reformulate the following question taking into account the context of the chat to perform keyword search and embeddings: