tests: green suite + CI smoke (92% cov, auth overrides, schema alignments)

[KyleSDeveloper]

What

• Add pytest suite (health, predict, predict_batch, version)
• Align request/response schemas with tests
• Lightweight stub model so API runs without external assets
• Add CI workflows (PR and main) with smoke check and coverage

Why

• Demonstrates production-readiness: tests, CI, auth gate, deterministic responses.

How to verify

• Actions tab → confirm CI jobs pass on this PR
• Locally: pytest --cov=serving_app --cov-report=term-missing (16 passed)

Notes

• API key is optional in CI smoke via API_KEY="" env.