Add Client with response wrapper, request helpers, and full middleware assembly

Implements the top-level httpx.Client that composes the full chain:
  Logging → User Middlewares → Retry → Circuit Breaker → Balancer → Transport

- Response wrapper with JSON/XML/Bytes decoding and body caching
- NewJSONRequest helper with Content-Type and GetBody support
- Functional options: WithBaseURL, WithTimeout, WithRetry, WithEndpoints, etc.
- Integration tests covering retry, balancing, error mapping, and JSON round-trips

request_test.go

@@ -0,0 +1,78 @@
+package httpx_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx"
+)
+
+func TestNewJSONRequest(t *testing.T) {
+	t.Run("body is JSON encoded", func(t *testing.T) {
+		payload := map[string]string{"key": "value"}
+		req, err := httpx.NewJSONRequest(context.Background(), http.MethodPost, "http://example.com", payload)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		body, err := io.ReadAll(req.Body)
+		if err != nil {
+			t.Fatalf("reading body: %v", err)
+		}
+
+		var decoded map[string]string
+		if err := json.Unmarshal(body, &decoded); err != nil {
+			t.Fatalf("unmarshal: %v", err)
+		}
+		if decoded["key"] != "value" {
+			t.Errorf("decoded[key] = %q, want %q", decoded["key"], "value")
+		}
+	})
+
+	t.Run("content type is set", func(t *testing.T) {
+		req, err := httpx.NewJSONRequest(context.Background(), http.MethodPost, "http://example.com", "test")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		ct := req.Header.Get("Content-Type")
+		if ct != "application/json" {
+			t.Errorf("Content-Type = %q, want %q", ct, "application/json")
+		}
+	})
+
+	t.Run("GetBody works", func(t *testing.T) {
+		payload := map[string]int{"num": 123}
+		req, err := httpx.NewJSONRequest(context.Background(), http.MethodPost, "http://example.com", payload)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		if req.GetBody == nil {
+			t.Fatal("GetBody is nil")
+		}
+
+		// Read body first time
+		b1, err := io.ReadAll(req.Body)
+		if err != nil {
+			t.Fatalf("reading body: %v", err)
+		}
+
+		// Get a fresh body
+		body2, err := req.GetBody()
+		if err != nil {
+			t.Fatalf("GetBody(): %v", err)
+		}
+		b2, err := io.ReadAll(body2)
+		if err != nil {
+			t.Fatalf("reading body2: %v", err)
+		}
+
+		if string(b1) != string(b2) {
+			t.Errorf("GetBody returned different data: %q vs %q", b1, b2)
+		}
+	})
+}