fix(vpc): detect subnet with TGW route as PRIVATE_WITH_EGRESS

[bruecktech]

When I do Vpc.fromLookup in a VPC that has subnets with a default route to a TransitGateway the subnet is detected as PRIVATE_ISOLATED instead of PRIVATE_WITH_EGRESS
This PR adds the detection of subnets with TGW routes as PRIVATE_WITH_EGRESS instead of PRIVATE_ISOLATED.

This is potentially a breaking change depending on what is the expected behaviour. To me it seemed rather missed accidentally given that this previous PR mentions that PRIVATE_WITH_EGRESS was introduced also for Transit Gateways.

Closes #25626

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[gitpod-io]

[aws-cdk-automation]

The pull request linter has failed. See the aws-cdk-automation comment below for failure reasons. If you believe this pull request should receive an exemption, please comment and provide a justification.

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[bruecktech]

Exemption Request: I could not find any existing integration tests specifically for the vpc context provider. Please let me know in case I missed something

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

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Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).