feat: funciones puras HTTP — HTTPMiddlewareChain y HTTPCORSMiddleware

- HTTPMiddlewareChain: compone N middlewares preservando el orden (el primero es el mas externo)
- HTTPCORSMiddleware: genera Middleware con headers CORS configurables, maneja OPTIONS preflight con 204

Ambas son puras (sin I/O) y testeadas con httptest.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/http_middleware_chain_test.go

@@ -0,0 +1,81 @@
+package infra
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestHTTPMiddlewareChain(t *testing.T) {
+	t.Run("sin middlewares pasa al handler final", func(t *testing.T) {
+		chain := HTTPMiddlewareChain()
+		handler := chain(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+			w.Write([]byte("final"))
+		}))
+
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest("GET", "/", nil)
+		handler.ServeHTTP(rec, req)
+
+		if rec.Code != http.StatusOK {
+			t.Errorf("got status %d, want %d", rec.Code, http.StatusOK)
+		}
+		if rec.Body.String() != "final" {
+			t.Errorf("got body %q, want %q", rec.Body.String(), "final")
+		}
+	})
+
+	t.Run("un middleware se aplica correctamente", func(t *testing.T) {
+		addHeader := func(next http.Handler) http.Handler {
+			return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.Header().Set("X-Test", "applied")
+				next.ServeHTTP(w, r)
+			})
+		}
+
+		chain := HTTPMiddlewareChain(Middleware(addHeader))
+		handler := chain(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}))
+
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest("GET", "/", nil)
+		handler.ServeHTTP(rec, req)
+
+		if rec.Header().Get("X-Test") != "applied" {
+			t.Errorf("got X-Test=%q, want %q", rec.Header().Get("X-Test"), "applied")
+		}
+	})
+
+	t.Run("dos middlewares se aplican en orden izquierda-derecha", func(t *testing.T) {
+		order := []string{}
+
+		first := Middleware(func(next http.Handler) http.Handler {
+			return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				order = append(order, "first")
+				next.ServeHTTP(w, r)
+			})
+		})
+		second := Middleware(func(next http.Handler) http.Handler {
+			return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				order = append(order, "second")
+				next.ServeHTTP(w, r)
+			})
+		})
+
+		chain := HTTPMiddlewareChain(first, second)
+		handler := chain(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			order = append(order, "handler")
+			w.WriteHeader(http.StatusOK)
+		}))
+
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest("GET", "/", nil)
+		handler.ServeHTTP(rec, req)
+
+		if len(order) != 3 || order[0] != "first" || order[1] != "second" || order[2] != "handler" {
+			t.Errorf("got order %v, want [first second handler]", order)
+		}
+	})
+}