fix(middleware): track actual response size

Fixes incorrect ResponseSize reporting when response body capture is truncated or skipped.

Author: georgidhristov

DebugProbe.AspNetCore.Tests/Middleware/ResponseBodyCaptureTests.cs

@@ -41,6 +41,7 @@ public async Task Handles_large_response_within_limit()
         await app.Client.GetAsync("/large");
 
         Assert.Equal(body, app.SingleEntry.ResponseBody);
+        Assert.Equal(body.Length, app.SingleEntry.ResponseSize);
     }
 
     [Fact]
@@ -54,6 +55,7 @@ public async Task Handles_binary_response_safely()
         await app.Client.GetAsync("/binary");
 
         Assert.Equal("[Body not captured: non-text content]", app.SingleEntry.ResponseBody);
+        Assert.Equal(4, app.SingleEntry.ResponseSize);
     }
 
     [Fact]
@@ -69,5 +71,6 @@ public async Task Validates_bounded_response_capture_behavior()
         var entry = app.SingleEntry;
         Assert.Equal(body, await response.Content.ReadAsStringAsync());
         Assert.Equal("[Body too large]", entry.ResponseBody);
+        Assert.Equal(body.Length, entry.ResponseSize);
     }
 }

DebugProbe.AspNetCore/Middleware/DebugProbeMiddleware.cs

@@ -125,7 +125,7 @@ public async Task Invoke(HttpContext context, DebugEntryStore store)
 
             entry.RequestSize = context.Request.ContentLength ?? Encoding.UTF8.GetByteCount(requestBody);
 
-            entry.ResponseSize = Encoding.UTF8.GetByteCount(responseBody);
+            entry.ResponseSize = responseCapture.TotalBytesWritten;
 
             entry.RequestHeaders =
                 context.Request.Headers.ToDictionary(