test: add Phase 1 integration tests for basic proxy

6 tests proving the scaffold works end-to-end:
- chat completion proxied through gateway to mock backend
- /health endpoint with healthy node
- /v1/models returns seeded model list
- 404 for unknown model
- 404 when no healthy nodes available
- 400 when request body missing model field

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/Cargo.toml

@@ -23,3 +23,6 @@ futures.workspace = true
 tokio-stream.workspace = true
 eventsource-stream.workspace = true
 bytes = "1"
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["test-util"] }