kiro power: error-handling succeeded failed methods

[yaythomas]

Correct fix, and verified against batch-result.ts:18,25: succeeded() and failed() are methods. All four occurrences in concurrent-operations.md are fixed properly.

Blocking on this one small expansion: the same bug exists twice more in aws-lambda-durable-functions-power/steering/error-handling.md at L399 and L407:

// L399
failures: results.failed.map(f => ({
  index: f.index,
  error: f.error?.message
}))

// L407
const failedItems = results.failed.map(f => event.items[f.index]);

Both need the same .failed() method-call treatment. Would you mind folding them into this PR so we don't ship a partial fix across two pages? The scope stays small.

Originally posted by @yaythomas in #177 (review)