Preserve default fallback result when no decoder plugin matches

[Copilot]

MessageDecoder.decode() initialized a useful fallback result for unmatched messages, but then replaced it with each plugin's unsuccessful return. As a result, callers could receive the last plugin's partial "unknown" response instead of the intended top-level failure payload.

• Decoder fallback semantics

  • Keep the prebuilt default DecodeResult as the return value unless a plugin actually decodes the message.
  • Stop replacing the fallback with intermediate plugin failures, so unmatched messages retain:
    • error: 'No known decoder plugin for this message'
    • decoder: { name: 'none', type: 'none', decodeLevel: 'none' }
    • the original message
    • full remaining.text
    • formatted.description: 'Not Decoded'

• Regression coverage

  • Add a MessageDecoder test for a registered label (44) with garbage payload that no plugin can decode.
  • Assert that the returned object is the default unmatched-message result rather than the last attempted plugin result.

• Behavioral change

  let result = defaultNoMatchResult;
  
  for (const plugin of usablePlugins) {
    const pluginResult = plugin.decode(message, options);
    if (pluginResult.decoded) {
      result = pluginResult;
      break;
    }
  }
  
  return result;