This terraform illustrates how to fetch the role definition ID via its common name (e.g. "Reader"), when using AzAPI.

RoleDefinitionByName.tf

terraform {
  required_providers {
    azapi = {
      source  = "Azure/azapi"
      version = "2.1.0"
    }
  }
}

variable "subscription_id" {
  description = "The subscription ID to query for role definitions."
  type        = string
}

variable "role_names" {
  description = "Role Definition names to query."
  type        = set(string)
  default     = ["Reader", "Key Vault Secrets Officer"]
}

data "azapi_resource_list" "role_definition" {
  for_each = var.role_names

  type      = "Microsoft.Authorization/roleDefinitions@2022-05-01-preview"
  parent_id = "/subscriptions/${var.subscription_id}"
  query_parameters = {
    "$filter" = ["roleName eq '${each.key}'"]
  }
  response_export_values = {
    "values" = "value[].{id: id}"
  }
}

locals {
  role_definition_map = {
    for role_name, role_definition in data.azapi_resource_list.role_definition : role_name => one(role_definition.output.values).id
  }
}

output "role_definitions" {
  value = local.role_definition_map
}

Suggestion;-

output "role_definition" { value = { for k, v in data.azapi_resource_list.role_definition : k => v.output.value[0].name } }

That's fair. I'd probably wrap it in a one() function to check only one was returned.