[AWS::SageMaker::EndpointConfig] - [Coverage] - Add ScaleInPolicy to ManagedInstanceScaling

### Name of the resource

AWS::SageMaker::EndpointConfig

### Resource name

_No response_

### Description

The SageMaker API added ScaleInPolicy to ProductionVariantManagedInstanceScaling (released in AWS SDK JS v3.1020.0). This allows configuring scale-in behavior (e.g. LinearScaleInConfig) for managed instance scaling on real-time endpoints.
The CloudFormation resource AWS::SageMaker::EndpointConfig exposes ManagedInstanceScaling under ProductionVariants, but currently only supports:

Status
MinInstanceCount
MaxInstanceCount

ScaleInPolicy is missing, making it impossible to configure this behavior via CloudFormation or CDK without using escape hatches (addOverride).

### Other Details

API reference: https://docs.aws.amazon.com/sagemaker/latest/APIReference/API_ProductionVariantManagedInstanceScaling.html#sagemaker-Type-ProductionVariantManagedInstanceScaling-ScaleInPolicy
SDK release: https://github.com/aws/aws-sdk-js-v3/releases/tag/v3.1020.0

Start with the AWS::SageMaker::EndpointConfig resource coverage and compare its ProductionVariants ManagedInstanceScaling properties with the linked SageMaker API reference. Add support for ScaleInPolicy, including LinearScaleInConfig, so the property can be configured through CloudFormation and CDK without an override.

Written by the indexing model from the issue text.