Switch EKS tests to SPOT instances

[Copilot]

All EKS test clusters were using ON_DEMAND instances. Switch to SPOT across all test variants to reduce costs, consistent with the approach already used in aws/modules/fixtures/fixtures.default.eks.tfvars.

Changes

• eks-single-region + eks-single-region-irsa cluster.tf — Added np_capacity_type = "SPOT" and expanded np_instance_types to multiple types for better SPOT availability:

  np_instance_types = ["m6i.xlarge", "m5.xlarge", "m5d.xlarge"]
  np_capacity_type  = "SPOT"

• aws-kubernetes-eks-dual-region-create action — Injected SPOT settings into the generated terraform-cluster.tfvars:

  np_capacity_type  = "SPOT"
  np_instance_types = ["m6i.xlarge", "m5.xlarge", "m5d.xlarge"]

• eks-dual-region Go test helpers — Re-enabled previously commented-out SPOT configuration in TestSetupTerraform and TestTeardownTerraform (had been disabled due to flakiness).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Switch to SPOT insances for tests</issue_title>
<issue_description>After the merge, switch to SPOT instances in the tests (similar to https://github.com/camunda/camunda-tf-eks-module/blob/main/modules/fixtures/fixtures.default.eks.tfvars)</issue_description>

<agent_instructions>Do it for all tests of the repo</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Switch to SPOT insances for tests #109

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[Langleu]

the agent took the instructions very literal 😁.

[leiicamundi]

@Langleu the result of this little experience around agents is not successful, it does work only for small things

[Langleu]

I mean it's not that wrong, it just applied it to everything.