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

response.go

@@ -0,0 +1,99 @@
+package httpx
+
+import (
+	"bytes"
+	"encoding/json"
+	"encoding/xml"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+// Response wraps http.Response with convenience methods.
+type Response struct {
+	*http.Response
+	body []byte
+	read bool
+}
+
+func newResponse(resp *http.Response) *Response {
+	return &Response{Response: resp}
+}
+
+// Bytes reads and returns the entire response body.
+// The body is cached so subsequent calls return the same data.
+func (r *Response) Bytes() ([]byte, error) {
+	if r.read {
+		return r.body, nil
+	}
+	defer r.Body.Close()
+	b, err := io.ReadAll(r.Body)
+	if err != nil {
+		return nil, err
+	}
+	r.body = b
+	r.read = true
+	return b, nil
+}
+
+// String reads the response body and returns it as a string.
+func (r *Response) String() (string, error) {
+	b, err := r.Bytes()
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}
+
+// JSON decodes the response body as JSON into v.
+func (r *Response) JSON(v any) error {
+	b, err := r.Bytes()
+	if err != nil {
+		return fmt.Errorf("httpx: reading response body: %w", err)
+	}
+	if err := json.Unmarshal(b, v); err != nil {
+		return fmt.Errorf("httpx: decoding JSON: %w", err)
+	}
+	return nil
+}
+
+// XML decodes the response body as XML into v.
+func (r *Response) XML(v any) error {
+	b, err := r.Bytes()
+	if err != nil {
+		return fmt.Errorf("httpx: reading response body: %w", err)
+	}
+	if err := xml.Unmarshal(b, v); err != nil {
+		return fmt.Errorf("httpx: decoding XML: %w", err)
+	}
+	return nil
+}
+
+// IsSuccess returns true if the status code is in the 2xx range.
+func (r *Response) IsSuccess() bool {
+	return r.StatusCode >= 200 && r.StatusCode < 300
+}
+
+// IsError returns true if the status code is 4xx or 5xx.
+func (r *Response) IsError() bool {
+	return r.StatusCode >= 400
+}
+
+// Close drains and closes the response body.
+func (r *Response) Close() error {
+	if r.read {
+		return nil
+	}
+	_, _ = io.Copy(io.Discard, r.Body)
+	return r.Body.Close()
+}
+
+// BodyReader returns a reader for the response body.
+// If the body has already been read via Bytes/String/JSON/XML,
+// returns a reader over the cached bytes.
+func (r *Response) BodyReader() io.Reader {
+	if r.read {
+		return bytes.NewReader(r.body)
+	}
+	return r.Body
+}