Allow loading any document in some assistants (#508)

Co-authored-by: krut_ni <nils.kruthoff@dlr.de>
Co-authored-by: Thorsten Sommer <mail@tsommer.org>

app/MindWork AI Studio/Tools/ContentStreamMetadataJsonConverter.cs

@@ -0,0 +1,32 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace AIStudio.Tools;
+
+public sealed class ContentStreamMetadataJsonConverter : JsonConverter<ContentStreamSseMetadata>
+{
+    public override ContentStreamSseMetadata? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        using var jsonDoc = JsonDocument.ParseValue(ref reader);
+        var root = jsonDoc.RootElement;
+        var rawText = root.GetRawText();
+        
+        var propertyName = root.EnumerateObject()
+            .Select(p => p.Name)
+            .FirstOrDefault();
+
+        return propertyName switch
+        {
+            "Text" => JsonSerializer.Deserialize<ContentStreamTextMetadata?>(rawText, options),
+            "Pdf" => JsonSerializer.Deserialize<ContentStreamPdfMetadata?>(rawText, options),
+            "Spreadsheet" => JsonSerializer.Deserialize<ContentStreamSpreadsheetMetadata?>(rawText, options),
+            "Presentation" => JsonSerializer.Deserialize<ContentStreamPresentationMetadata?>(rawText, options),
+            "Image" => JsonSerializer.Deserialize<ContentStreamImageMetadata?>(rawText, options),
+            "Document" => JsonSerializer.Deserialize<ContentStreamDocumentMetadata?>(rawText, options),
+            
+            _ => null
+        };
+    }
+
+    public override void Write(Utf8JsonWriter writer, ContentStreamSseMetadata value, JsonSerializerOptions options) => JsonSerializer.Serialize(writer, value, value.GetType(), options);
+}