fix(ecs): require task pidMode for Linux-based Fargate tasks, not host

[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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Your pull request must be based off of a branch in a personal account (not an organization owned account, and not the main branch). You must also have the setting enabled that allows the CDK team to push changes to your branch (this setting is enabled by default for personal accounts, and cannot be enabled for organization owned accounts). The reason for this is that our automation needs to synchronize your branch with our main after it has been approved, and we cannot do that if we cannot push to your branch.

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #30020. Mergify cannot evaluate rules on this PR. ⚠️

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AWS CodeBuild CI Report

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