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

error_test.go

@@ -0,0 +1,90 @@
+package httpx_test
+
+import (
+	"errors"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx"
+)
+
+func TestError(t *testing.T) {
+	t.Run("formats without endpoint", func(t *testing.T) {
+		inner := errors.New("connection refused")
+		e := &httpx.Error{
+			Op:  "Get",
+			URL: "http://example.com/api",
+			Err: inner,
+		}
+
+		want := "httpx: Get http://example.com/api: connection refused"
+		if got := e.Error(); got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	})
+
+	t.Run("formats with endpoint different from url", func(t *testing.T) {
+		inner := errors.New("timeout")
+		e := &httpx.Error{
+			Op:       "Do",
+			URL:      "http://example.com/api",
+			Endpoint: "http://node1.example.com/api",
+			Err:      inner,
+		}
+
+		want := "httpx: Do http://example.com/api (endpoint http://node1.example.com/api): timeout"
+		if got := e.Error(); got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	})
+
+	t.Run("formats with endpoint same as url", func(t *testing.T) {
+		inner := errors.New("not found")
+		e := &httpx.Error{
+			Op:       "Get",
+			URL:      "http://example.com/api",
+			Endpoint: "http://example.com/api",
+			Err:      inner,
+		}
+
+		want := "httpx: Get http://example.com/api: not found"
+		if got := e.Error(); got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	})
+
+	t.Run("unwrap returns inner error", func(t *testing.T) {
+		inner := errors.New("underlying")
+		e := &httpx.Error{Op: "Get", URL: "http://example.com", Err: inner}
+
+		if got := e.Unwrap(); got != inner {
+			t.Errorf("Unwrap() = %v, want %v", got, inner)
+		}
+
+		if !errors.Is(e, inner) {
+			t.Error("errors.Is should find the inner error")
+		}
+	})
+}
+
+func TestSentinelErrors(t *testing.T) {
+	t.Run("ErrRetryExhausted", func(t *testing.T) {
+		if httpx.ErrRetryExhausted == nil {
+			t.Fatal("ErrRetryExhausted is nil")
+		}
+		if httpx.ErrRetryExhausted.Error() == "" {
+			t.Fatal("ErrRetryExhausted has empty message")
+		}
+	})
+
+	t.Run("ErrCircuitOpen", func(t *testing.T) {
+		if httpx.ErrCircuitOpen == nil {
+			t.Fatal("ErrCircuitOpen is nil")
+		}
+	})
+
+	t.Run("ErrNoHealthy", func(t *testing.T) {
+		if httpx.ErrNoHealthy == nil {
+			t.Fatal("ErrNoHealthy is nil")
+		}
+	})
+}