test: characterize single Grafana module rendering

[lokeshmuvva]

Summary

First step of AWS ECS Fargate support. This PR is groundwork only — it adds test coverage and changes no runtime behavior.

Adds tests/test_template_rendering.py, a characterization test asserting the Grafana module is emitted exactly once by the cloud_run template. This locks in the fix from #62 so the AWS work can't silently regress it.

Why this first

The AWS plan requires existing GCP behavior to stay green throughout. There was no test guarding that, so this establishes the baseline before any provider abstraction lands.

The test defines its fixture inline rather than adding tests/conftest.py, deliberately: conftest.py is one of the files #63 also adds, and there's no reason to contest it for one fixture.

Known gap, fixed in the next commit

cli.py selects mlflow_main.tf.j2 whenever the stack contains MLflow — which the documented stack always does — so this test currently guards a template the documented stack doesn't render. The follow-up extends the same assertion to mlflow_main.tf.j2. Flagging it here rather than quietly shipping partial coverage.

What's coming on this branch

AWS support lands incrementally on this PR: provider config validation and seams, then the AWS Terraform foundation (VPC/RDS/S3/ECS), the database bootstrap container, and MLflow/FastAPI/Grafana on Fargate. Tasks touching cli.py, doctor.py, and infracost.py are sequenced behind #64 to avoid conflicting with the Infracost v2 work.

Test plan

• pytest tests/ -v — 64 passed
• Rebased onto current main, conflict-free, suite still green
• Reviewer confirms the characterization matches intended cloud_run behavior