Eliminate trust policy race by migrating emr_eks test to Pod Identity

[ramitkataria]

The test shares one EMR job execution role across runs while each run creates its own EKS cluster. With IRSA, every run had to rewrite the shared role's trust policy to federate its own cluster's OIDC provider, and pruning old statements to stay under the trust policy size limit meant concurrent runs deleted each other's trust entries, failing jobs with AssumeRoleWithWebIdentity AccessDenied. IAM offers no compare-and-swap on trust policies, so this race cannot be fixed while runs mutate shared IAM state.

EKS Pod Identity (supported by EMR on EKS since emr-7.3.0) removes the shared mutable state: the role keeps one static trust policy and each run creates Pod Identity associations scoped to its own cluster, also avoiding the 4096-char trust policy cap that limits IRSA to roughly a dozen clusters per role.

Pod Identity is also AWS's recommended mechanism for pod credentials:

Unless you have specific usecases for IRSA, we recommend you use
EKS Pod Identities when using EKS.

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