feat: improve ResendRetrieveError to match all API error shapes

[asielcabrera]

Description

The ResendRetrieveError model may not cover all error response shapes returned by the Resend API.

Current model

public struct ResendRetrieveError: Codable, Sendable, Error {
    public var statusCode: Int
    public var message: String
    public var name: String
}

Verified API responses

// Validation error
{"statusCode":400,"message":"Your plan includes 3 segments.","name":"validation_error"}

// Domain error
{"statusCode":403,"message":"domain has been registered already","name":"validation_error"}

// Not found
{"statusCode":422,"message":"Email is not scheduled","name":"validation_error"}

// Application error (500)
{"message":"Something went wrong","statusCode":500,"name":"application_error"}

The model currently works for all observed cases, but we should add tests for edge cases and possibly add an errors array field for batch errors.