feat: funciones Go — core (cron, join_by_key, validate_struct), datascience (pivot, diff_entities), infra (http, cache, cron_ticker)

Nuevas funciones Go con tests en tres dominios:
- core: parse_cron_expr, next_cron_time, join_by_key, validate_struct_fields + tipo CronSchedule
- datascience: pivot (tabla dinámica), diff_entities (comparación de entidades)
- infra: http_get_json, http_post_json, http_download_file, cache_to_sqlite, cron_ticker

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/infra/http_post_json_test.go

@@ -0,0 +1,67 @@
+package infra
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestHttpPostJSON(t *testing.T) {
+	t.Run("httptest.Server recibe body correcto", func(t *testing.T) {
+		received := make(chan map[string]any, 1)
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			var body map[string]any
+			data, _ := io.ReadAll(r.Body)
+			json.Unmarshal(data, &body)
+			received <- body
+			w.Header().Set("Content-Type", "application/json")
+			w.Write([]byte(`{"ok": true}`))
+		}))
+		defer srv.Close()
+
+		_, err := HttpPostJSON(srv.URL, map[string]any{"name": "Alice", "score": 100}, nil, 5*time.Second)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		body := <-received
+		if body["name"] != "Alice" {
+			t.Errorf("name not received correctly, got: %v", body["name"])
+		}
+	})
+
+	t.Run("Status 201 → exito", func(t *testing.T) {
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusCreated)
+			w.Write([]byte(`{"id": 42}`))
+		}))
+		defer srv.Close()
+
+		result, err := HttpPostJSON(srv.URL, map[string]any{"x": 1}, nil, 5*time.Second)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if result["id"] != float64(42) {
+			t.Errorf("got id=%v, want 42", result["id"])
+		}
+	})
+
+	t.Run("Status 500 → error con body parcial", func(t *testing.T) {
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			http.Error(w, "internal server error details", http.StatusInternalServerError)
+		}))
+		defer srv.Close()
+
+		_, err := HttpPostJSON(srv.URL, map[string]any{}, nil, 5*time.Second)
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+		if !strings.Contains(err.Error(), "500") {
+			t.Errorf("error should contain 500, got: %v", err)
+		}
+	})
+}