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>}