Fargate pidmode task

[CharlesHolmes]

Issue # (if applicable)

Closes #29995.

Reason for this change

Only the task option is allowed for [pidMode](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ecs-taskdefinition.html#cfn-ecs-taskdefinition-pidmode) on Linux-based Fargate tasks.

Description of changes

This PR builds on the changes introduced in #29670 but fixes the handling of pidMode so that it matches the behavior allowed by CloudFormation and described in the AWS User Guide.

Description of how you validated changes

Updated the existing tests so that task is the only allowable pidMode setting if a Fargate task's OS is Linux-based.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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[mergify]

⚠️ The sha of the head commit of this PR conflicts with #30019. Mergify cannot evaluate rules on this PR. ⚠️

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: e80a7ac
• Result: SUCCEEDED
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