test(integration): reorganize test structure and add runner script

- Move integration tests and mock upstream resources from `integration/` to `test/` directory.
- Add `test/integration.sh` script to orchestrate environment setup and test execution.
- Update build context in `docker-compose.integration.yml` to match new structure.
- Add documentation for local development and integration testing workflows in README.

test/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)
+	}
+}