Emails resource – wraps the /emails Resend REST endpoints.

Mirrors the interface of the official SDKs (Node, Python, Java, Go, etc.):

  emails/send!        POST   /emails
  emails/send-batch!  POST   /emails/batch
  emails/get!         GET    /emails/:id
  emails/update!      PATCH  /emails/:id
  emails/cancel!      POST   /emails/:id/cancel

All functions return {:data <parsed-map> :error nil} on success or
{:data nil :error <error-map>} on failure, so callers never need a
try/catch for normal API errors. Bang (!) variants throw instead.

References:
- Resend send email API: https://resend.com/docs/api-reference/emails/send-email

Public entry point for the resend-clojure SDK.

Most users only need this namespace and the resource namespaces
(e.g. resend.emails).

Example:
  (require '[resend.core   :as resend])
  (require '[resend.emails :as emails])

  (def client (resend/create-client {:api-key (System/getenv "RESEND_API_KEY")}))

  (emails/send client
    {:from    "Acme <no-reply@acme.com>"
     :to      "user@example.com"
     :subject "Hello"
     :html    "<strong>Hello!</strong>"})

Constructs and holds the SDK client configuration.

Following the pattern used by resend-node, resend-java, and other official
SDKs, a single client record is created once (with the API key) and then
threaded through every resource namespace call.

Defines the IHttpAdapter protocol and the default hato-based implementation.

Separating transport from business logic allows consumers to inject a
MockAdapter in tests or swap in any alternative HTTP client without
touching SDK namespaces.

A MockAdapter that avoids real HTTP calls in unit tests.

Pattern: record captures the canned responses you want to return,
plus a `calls` atom that logs every request for assertion.

Converts raw HTTP responses into the {:data … :error …} envelope used by the SDK.
Success: 2xx  → {:data <kebab-case map>  :error nil}
Failure: 4xx+ → {:data nil :error <kebab-case map>}