Structured error for AWS SES inbound S3-object fetch failures.

Raised when the SES receipt-rule S3 action object cannot be retrieved and fed into MailglassInbound.MIME.parse/1. Mirrors the shape of MailglassInbound.MIMEError / Mailglass.ConfigError — a closed :type atom set, a [:type, :message, :cause, :context] defexception, and a Jason.Encoder derivation that excludes :cause so raw S3/error fragments never leak into serialized output (logs, telemetry, admin). Match on the struct and :type, never on the message string.

This struct is package-local: it does NOT implement the core Mailglass.Error behaviour and does NOT join Mailglass.Error's @type union. It is a plain defexception with a __types__/0 contract helper.

Types

• :s3_object_not_ready — the bounded GetObject retry was exhausted before the object became readable. This is the transient path: the handler does NOT ack, so SNS redelivers and the dedupe layer is the real safety net (D-46-16).
• :s3_fetch_failed — a non-retryable S3 error (access denied, bucket missing, malformed key, gateway/exit failure). Retrying will not help.

See docs/api_stability.md (mailglass_inbound) for the locked :type atom set. Adding a value requires a CHANGELOG entry + @since annotation (minor version bump); removing a value requires a major version bump.