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/service/sync_test.go

@@ -0,0 +1,58 @@
+package service
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/alicebob/miniredis/v2"
+	"github.com/ez-api/ez-api/internal/model"
+	"github.com/redis/go-redis/v9"
+)
+
+func TestSyncProvider_WritesSnapshotAndRouting(t *testing.T) {
+	mr := miniredis.RunT(t)
+	rdb := redis.NewClient(&redis.Options{Addr: mr.Addr()})
+
+	svc := NewSyncService(rdb)
+
+	p := &model.Provider{
+		Name:           "p1",
+		Type:           "vertex-express",
+		Group:          "default",
+		Models:         "gemini-3-pro-preview",
+		Status:         "active",
+		AutoBan:        true,
+		GoogleProject:  "",
+		GoogleLocation: "global",
+	}
+	p.ID = 42
+
+	if err := svc.SyncProvider(p); err != nil {
+		t.Fatalf("SyncProvider: %v", err)
+	}
+
+	raw := mr.HGet("config:providers", "42")
+	if raw == "" {
+		t.Fatalf("expected config:providers hash entry")
+	}
+
+	var snap map[string]any
+	if err := json.Unmarshal([]byte(raw), &snap); err != nil {
+		t.Fatalf("invalid snapshot json: %v", err)
+	}
+	if snap["google_location"] != "global" {
+		t.Fatalf("expected google_location=global, got %v", snap["google_location"])
+	}
+
+	routeKey := "route:group:default:gemini-3-pro-preview"
+	if !mr.Exists(routeKey) {
+		t.Fatalf("expected routing key %q to exist", routeKey)
+	}
+	ok, err := mr.SIsMember(routeKey, "42")
+	if err != nil {
+		t.Fatalf("SIsMember: %v", err)
+	}
+	if !ok {
+		t.Fatalf("expected provider id 42 in routing set %q", routeKey)
+	}
+}