Build with confidence: test keys and safe retries

Two things make building on the Avox API calmer.

• NewTest keys (they start with avx_test_) run every check but never send. Responses say they're a test, so your code can tell.
• NewSpecial test numbers return each kind of error on purpose, so you can see how your system handles them.
• NewSend an Idempotency-Key header with a request, and a retry with the same key won't send the message twice. A replay says so in the response.
• ImprovedEvery request your keys make is listed on the API logs page, with a CSV export.

When something does go wrong, each error comes with a stable code, a message and a link to the docs page that explains it.