docs(tests): how to run unit/codegen/smoke suites

Author: joaodaher

tests/README.md

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+Testing layout
+
+- smoke/
+  - Cloud Build pipeline and Python runner that deploys the example app and Workflows, then invokes workflows end-to-end and asserts final results.
+
+- unit/
+  - FastAPI TestClient tests that call each step endpoint under /steps and validate Pydantic contracts and route registration.
+
+- codegen/
+  - CLI-driven codegen tests that generate YAML and compare full-file snapshots under tests/fixtures/yaml.
+
+Notes
+- Set GOOGLE_CLOUD_PROJECT (or configure gcloud) to run smoke tests locally.
+- On first snapshot run, snapshot files will be created under tests/fixtures/yaml; commit them and re-run.
+