Add bounded diagnostic context

An ErrorContextProvider should expose facts needed to diagnose a class of failure, not a snapshot of the entire application.

final class QueueContextProvider implements ErrorContextProvider {
  const QueueContextProvider(this.queue);

  final WorkQueue queue;

  @override
  ErrorContextSection collect(
    ErrorOccurrence occurrence,
    ErrorRuntimeEnvironment environment,
  ) => ErrorContextSection(
    name: 'queue',
    fields: <String, Object?>{
      'pending': queue.pendingCount,
      'capacity': queue.capacity,
    },
  );
}

Good context has bounded size, stable names, and a clear diagnostic purpose. Prefer counts, status codes, retry numbers, operation names, and non-personal correlation IDs.

Do not collect credentials, cookies, request bodies, route arguments, free-form user text, complete domain state, or opaque objects whose toString() output is uncontrolled.