Add dd0c/route integration tests: proxy engine with wiremock

- Forward request to upstream and verify response passthrough
- Telemetry event emission (org_id, model, latency, status)
- Low-complexity routing: gpt-4o → gpt-4o-mini with strategy=cheapest
- Upstream error passthrough (429 rate limit)
- Invalid JSON → 400 Bad Request
- Health endpoint returns 200

products/01-llm-cost-router/tests/integration/mod.rs

@@ -0,0 +1 @@
+mod proxy_test;