test: add comprehensive unit tests for provider, middleware, and sync service

- Add TESTING.md documentation explaining unit test conventions and integration testing setup
- Add miniredis and sqlite dependencies to go.mod for in-memory testing
- Add provider_handler_test.go ensuring Vertex providers default google_location to "global"
- Add request_id_test.go verifying request ID generation and header preservation
- Add sync_test.go validating Redis snapshot writes and routing key generation
- Update README.md with testing section referencing new documentation

internal/api/provider_handler_test.go

@@ -0,0 +1,105 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/alicebob/miniredis/v2"
+	"github.com/ez-api/ez-api/internal/dto"
+	"github.com/ez-api/ez-api/internal/model"
+	"github.com/ez-api/ez-api/internal/service"
+	"github.com/gin-gonic/gin"
+	"github.com/redis/go-redis/v9"
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+)
+
+func newTestHandler(t *testing.T) (*Handler, *gorm.DB) {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+
+	// Use a unique in-memory DB per test to avoid cross-test interference.
+	dsn := fmt.Sprintf("file:%s?mode=memory&cache=shared", t.Name())
+	db, err := gorm.Open(sqlite.Open(dsn), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	if err := db.AutoMigrate(&model.Provider{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+
+	mr := miniredis.RunT(t)
+	rdb := redis.NewClient(&redis.Options{Addr: mr.Addr()})
+	sync := service.NewSyncService(rdb)
+
+	return NewHandler(db, sync, nil), db
+}
+
+func TestCreateProvider_DefaultsVertexLocationGlobal(t *testing.T) {
+	h, _ := newTestHandler(t)
+
+	r := gin.New()
+	r.POST("/admin/providers", h.CreateProvider)
+
+	reqBody := dto.ProviderDTO{
+		Name:   "g1",
+		Type:   "vertex-express",
+		Group:  "default",
+		Models: []string{"gemini-3-pro-preview"},
+	}
+	b, _ := json.Marshal(reqBody)
+
+	req := httptest.NewRequest(http.MethodPost, "/admin/providers", bytes.NewReader(b))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+	r.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusCreated {
+		t.Fatalf("expected 201, got %d body=%s", rr.Code, rr.Body.String())
+	}
+	var got model.Provider
+	if err := json.Unmarshal(rr.Body.Bytes(), &got); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if got.GoogleLocation != "global" {
+		t.Fatalf("expected google_location=global, got %q", got.GoogleLocation)
+	}
+}
+
+func TestUpdateProvider_DefaultsVertexLocationGlobalWhenMissing(t *testing.T) {
+	h, db := newTestHandler(t)
+
+	existing := &model.Provider{
+		Name:   "g2",
+		Type:   "vertex",
+		Group:  "default",
+		Models: "gemini-3-pro-preview",
+		Status: "active",
+	}
+	if err := db.Create(existing).Error; err != nil {
+		t.Fatalf("create provider: %v", err)
+	}
+
+	r := gin.New()
+	r.PUT("/admin/providers/:id", h.UpdateProvider)
+
+	req := httptest.NewRequest(http.MethodPut, fmt.Sprintf("/admin/providers/%d", existing.ID), bytes.NewReader([]byte(`{}`)))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+	r.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d body=%s", rr.Code, rr.Body.String())
+	}
+	var got model.Provider
+	if err := json.Unmarshal(rr.Body.Bytes(), &got); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if got.GoogleLocation != "global" {
+		t.Fatalf("expected google_location=global, got %q", got.GoogleLocation)
+	}
+}