fix: Update s3 extension logic to handle "__all__" (off-ticket)

[alexlescionok]

WHY

Observed behaviour: The extensions config only successfully applies when ensuring __all__ is in a list of strings, not as a string.

The terraform apply falls over if platform-config.yml contains:

services: __all__

Platform-helper schema validation shows we can add a string of __all__ OR a list of strings.

TESTING

Temporary locals for testing all logic in terraform console confirms the observed behaviour.

ECS-Services module has Terraform logic that uses contains function that expects a list of strings and not just a string.

Existing platform-helper logic does not catch this until we reach the terraform apply.

---

Checklist:

Title:

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Tasks:

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Reviewer Checklist

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[github-actions]

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[github-actions]

⚠️ Python Vulnerabilities detected: 1 unique issues.

Low:

• CVE-2026-1703 (fix: 26.0, severity: LOW, GHSA: GHSA-6vgw-5pg2-w6jp)