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/middleware/request_id_test.go

@@ -0,0 +1,57 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestRequestID_GeneratesAndEchoes(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	r := gin.New()
+	r.Use(RequestID())
+	r.GET("/x", func(c *gin.Context) {
+		c.String(200, c.GetHeader("X-Request-ID"))
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/x", nil)
+	rr := httptest.NewRecorder()
+	r.ServeHTTP(rr, req)
+
+	if rr.Code != 200 {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+	id := rr.Header().Get("X-Request-ID")
+	if id == "" {
+		t.Fatalf("expected X-Request-ID response header")
+	}
+	if rr.Body.String() != id {
+		t.Fatalf("expected body to echo X-Request-ID, got %q want %q", rr.Body.String(), id)
+	}
+}
+
+func TestRequestID_PreservesIncomingHeader(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	r := gin.New()
+	r.Use(RequestID())
+	r.GET("/x", func(c *gin.Context) {
+		c.String(200, c.GetHeader("X-Request-ID"))
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/x", nil)
+	req.Header.Set("X-Request-ID", "req-abc")
+	rr := httptest.NewRecorder()
+	r.ServeHTTP(rr, req)
+
+	if rr.Code != 200 {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+	if got := rr.Header().Get("X-Request-ID"); got != "req-abc" {
+		t.Fatalf("expected preserved X-Request-ID, got %q", got)
+	}
+}
+