v0.15.0

• Added: test mode. apiKeys.create({ mode: "test" }) mints a test key
  (prefixed mt_test_) whose sends are validated, recorded and webhook-emitting
  but never delivered, so CI can run against production Mailtea with your real
  code and your real webhook handler. A test key is not a data sandbox — it
  reads and writes your real contacts, templates, senders and webhooks. Only
  delivery is simulated.
• Added: mode on ListEmailsParams, so emails.list({ mode: "test" }) reads
  test-mode mail. There is no mixed view: a test key reads only test emails and
  a live key only live ones, and asking for the mode your key is not in is a 400
  rather than an empty page.
• Added: mode on EmailListItem, RetrievedEmail, CreatedApiKey and
  ApiKeyListItem, plus the EmailMode type — an open union like
  EmailStatus, so a value added server-side still types.
• Note: mode is never accepted on a send. The key decides.
• Reserved recipients on test.mailtea.email force an outcome: delivered@,
  bounced@, complained@, delayed@, failed@. The first to recipient
  decides; anything else is delivered.
• Added: MailteaError.reason — the API's second machine-readable
  discriminator, alongside code. Several routes answer with reason and no
  code (mode_not_available, test_recipient_in_live_mode,
  test_mode_daily_cap), and it was being dropped when the response was
  parsed, leaving those callers to string-match the message.

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Released from mailtea-app/mailtea@8c362fb.