fix: account ID based endpoints

Author: 0marperez

codegen/aws-sdk-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/endpoints/AwsBuiltins.kt

@@ -6,6 +6,7 @@ package aws.sdk.kotlin.codegen.endpoints
 
 object AwsBuiltins {
     const val ACCOUNT_ID = "AWS::Auth::AccountId"
+    const val ACCOUNT_ID_ENDPOINT_MODE = "AWS::Auth::AccountIdEndpointMode"
     const val USE_FIPS = "AWS::UseFIPS"
     const val USE_DUAL_STACK = "AWS::UseDualStack"
     const val S3_ACCELERATE = "AWS::S3::Accelerate"

codegen/aws-sdk-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/endpoints/BindAwsEndpointBuiltins.kt

@@ -106,6 +106,14 @@ fun renderBindAwsBuiltins(ctx: ProtocolGenerator.GenerationContext, writer: Kotl
                         AwsRuntimeTypes.Config.Endpoints.resolveAccountId,
                         AccountIdEndpointBuiltinCustomization.AccountIdEndpointModeProp.propertyName,
                     )
+
+                AwsBuiltins.ACCOUNT_ID_ENDPOINT_MODE -> {
+                    writer.write(
+                        "#L = config.#L.toString().lowercase()", // DDB endpoint rules assume lowercase value
+                        it.defaultName(),
+                        AccountIdEndpointBuiltinCustomization.AccountIdEndpointModeProp.propertyName,
+                    )
+                }
             }
         }
     }