fix(esg): honor the declared response charset in ReadBodyBoundedAsync with UTF-8 fallback (plan R2-06 T4)

src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs

@@ -466,6 +466,12 @@ public class ExternalSystemClient : IExternalSystemClient
     /// into store-and-forward would defeat the cap. Cancellation
     /// (<see cref="OperationCanceledException"/>) is allowed to propagate so the
     /// caller's ordered timeout/cancellation catch filters classify it.
+    /// <para>
+    /// N4: the accumulated bytes are decoded using the response's declared charset
+    /// (<c>Content-Type: …; charset=…</c>) via <see cref="ResolveEncoding"/> — parity
+    /// with the pre-bounded-read <c>ReadAsStringAsync</c>. A null, empty, or
+    /// unrecognized charset falls back to UTF-8.
+    /// </para>
     /// </summary>
     private static async Task<string> ReadBodyBoundedAsync(
         HttpContent content, string systemName, long maxBytes, CancellationToken token)
@@ -498,7 +504,33 @@ public class ExternalSystemClient : IExternalSystemClient
             }
         }
 
-        return Encoding.UTF8.GetString(buffer.GetBuffer(), 0, (int)buffer.Length);
+        // N4: honor the response's declared charset (parity with the pre-bounded-read
+        // ReadAsStringAsync). Unknown/invalid charset → UTF-8 fallback: mojibake beats
+        // failing the call over a cosmetic header.
+        return ResolveEncoding(content.Headers.ContentType?.CharSet)
+            .GetString(buffer.GetBuffer(), 0, (int)buffer.Length);
+    }
+
+    /// <summary>
+    /// Resolves the declared response charset to an <see cref="Encoding"/>, tolerating
+    /// the quoted form some servers emit (<c>charset="iso-8859-1"</c>). Null, empty,
+    /// or unrecognized values fall back to UTF-8.
+    /// </summary>
+    private static Encoding ResolveEncoding(string? charset)
+    {
+        if (string.IsNullOrWhiteSpace(charset))
+        {
+            return Encoding.UTF8;
+        }
+
+        try
+        {
+            return Encoding.GetEncoding(charset.Trim().Trim('"'));
+        }
+        catch (ArgumentException)
+        {
+            return Encoding.UTF8;
+        }
     }
 
     private static string Truncate(string value, int maxChars)