Use string.Concat instead of string.Join("" (#667)

Author: stephentoub

src/Custom/Responses/Items/Reasoning/ReasoningResponseItem.cs

@@ -19,8 +19,7 @@ public ReasoningResponseItem(string summaryText) : this(summaryParts: [new Reaso
     // CUSTOM: Added for convenience.
     public string GetSummaryText()
     {
-        return string.Join(
-            separator: string.Empty,
+        return string.Concat(
             values: SummaryParts.Select(part => (part as ReasoningSummaryTextPart)?.Text ?? string.Empty));
     }
 }

src/Custom/Responses/OpenAIResponse.cs

@@ -67,6 +67,6 @@ public string GetOutputText()
             .Select(item => item as MessageResponseItem)
             .SelectMany(message => message.Content.Where(contentPart => contentPart.Kind == ResponseContentPartKind.OutputText)
                 .Select(outputTextPart => outputTextPart.Text));
-        return outputTextSegments.Any() ? string.Join(string.Empty, outputTextSegments) : null;
+        return outputTextSegments.Any() ? string.Concat(outputTextSegments) : null;
     }
 }

tests/Responses/ResponsesTests.cs

@@ -252,7 +252,7 @@ public async Task StreamingResponses()
         }
         Assert.That(deltaTextSegments, Has.Count.GreaterThan(0));
         Assert.That(finalResponseText, Is.Not.Null.And.Not.Empty);
-        Assert.That(string.Join(string.Empty, deltaTextSegments), Is.EqualTo(finalResponseText));
+        Assert.That(string.Concat(deltaTextSegments), Is.EqualTo(finalResponseText));
     }
 
     [Test]