test(integration): add e2e testing suite and infrastructure

Introduces a comprehensive integration testing setup and local development environment.

Changes include:
- Add `docker-compose.yml` for local stack orchestration.
- Add `docker-compose.integration.yml` for the integration test environment.
- Create `mock-upstream` service to simulate external LLM provider responses.
- Implement Go-based end-to-end tests verifying control plane configuration and data plane routing.
- Add `integration_test.sh` for quick connectivity verification.

integration/mock-upstream/main.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"encoding/json"
+	"log"
+	"net/http"
+)
+
+func main() {
+	mux := http.NewServeMux()
+
+	mux.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("ok"))
+	})
+
+	mux.HandleFunc("/v1/chat/completions", func(w http.ResponseWriter, r *http.Request) {
+		resp := map[string]interface{}{
+			"id":     "mock-completion",
+			"object": "chat.completion",
+			"choices": []map[string]interface{}{
+				{
+					"index": 0,
+					"message": map[string]string{
+						"role":    "assistant",
+						"content": "hello from mock",
+					},
+					"finish_reason": "stop",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	})
+
+	mux.HandleFunc("/v1/models", func(w http.ResponseWriter, r *http.Request) {
+		resp := map[string]interface{}{
+			"object": "list",
+			"data": []map[string]string{
+				{"id": "mock-model", "object": "model", "owned_by": "mock"},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	})
+
+	log.Println("mock-upstream listening on :8082")
+	if err := http.ListenAndServe(":8082", mux); err != nil {
+		log.Fatalf("mock-upstream failed: %v", err)
+	}
+}