fix: delete workflow shouldn't require bucket_name_suffix

[sebastiancorrea81]

Summary

• delete.yaml and create.yaml share the build_context step, which required bucket_name_suffix unconditionally before deciding whether it was actually needed.
• On delete, the script already reuses the previously computed bucket_name from service attributes, so it never needs bucket_name_suffix — but the old check ran first and could fail the delete for no reason.
• Moved the required check inside the branch that actually computes a new bucket_name (i.e. only when there's no existing bucket_name to reuse).

Test plan

• bash -n aws-s3-bucket/scripts/aws/build_context (syntax check, passed locally)
• Create an S3 bucket service instance, then delete it, and confirm delete no longer fails on bucket_name_suffix
• Confirm create still fails with a clear error when bucket_name_suffix is missing