feat: reusable terraform-test workflow (INFRA-703)

[jgiuliani]

Adds a reusable workflow_call workflow to run tofu test. Part of INFRA-703 uniform AWS tagging.

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Note

Low Risk
CI-only addition with no application or infrastructure runtime changes; risk is limited to how future callers configure paths and OpenTofu version.

Overview
Introduces a new reusable GitHub Actions workflow .github/workflows/terraform-test.yml so other pipelines can run OpenTofu module tests without duplicating setup.

Callers can invoke it via workflow_call and pass opentofu_version (default 1.9.0), working_directory (module root), and test_directory (where .tftest.hcl files live, default tests). The job checks out the repo, installs OpenTofu with tofu_wrapper: false, runs tofu init -backend=false, then tofu test against the configured test directory.

This mirrors existing reusable Terraform CI workflows (e.g. lint/format) and supports INFRA-703-style uniform validation across modules; this PR does not wire any caller workflows yet.

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