Fix invalid catch block in StringUtilities (#49082)

Author: GrabYourPitchforks

src/Shared/ServerInfrastructure/StringUtilities.cs

@@ -683,9 +683,13 @@ private static bool IsValidHeaderString(string value)
             new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true).GetByteCount(value);
             return !value.Contains('\0');
         }
-        catch (DecoderFallbackException)
+        catch (ArgumentOutOfRangeException)
         {
-            return false;
+            return false; // 'value' too large to compute a UTF-8 byte count
+        }
+        catch (EncoderFallbackException)
+        {
+            return false; // 'value' cannot be converted losslessly to UTF-8
         }
     }