Update content decoding and validation logic

- Changed `PageNumber` in `Chunk` to nullable `int?` in `IContentDecoder` and updated related logic in `TextContentDecoder`.
- Revised citation rules in `ChatService` for stricter placement and formatting.
- Introduced `QuestionValidator` class with validation rules for `Question` model's `Text` property.

SqlDatabaseVectorSearch/ContentDecoders/IContentDecoder.cs

@@ -5,4 +5,4 @@ public interface IContentDecoder
     Task<IEnumerable<Chunk>> DecodeAsync(Stream stream, string contentType, CancellationToken cancellationToken = default);
 }
 
-public record class Chunk(int PageNumber, int IndexOnPage, string Content);
+public record class Chunk(int? PageNumber, int IndexOnPage, string Content);

SqlDatabaseVectorSearch/ContentDecoders/TextContentDecoder.cs

@@ -12,6 +12,6 @@ public class TextContentDecoder(IServiceProvider serviceProvider) : IContentDeco
         var content = await readStream.ReadToEndAsync(cancellationToken);
 
         var paragraphs = textChunker.Split(content);
-        return paragraphs.Select((text, index) => new Chunk(1, index, text)).ToList();
+        return paragraphs.Select((text, index) => new Chunk(null, index, text)).ToList();
     }
 }