Add retry transport with configurable backoff and Retry-After support

Implements retry middleware as a RoundTripper wrapper:
- Exponential and constant backoff strategies with jitter
- RFC 7231 Retry-After header parsing (seconds and HTTP-date)
- Default policy retries idempotent methods on 429/5xx and network errors
- Body restoration via GetBody, context cancellation, response body cleanup

retry/retry_after_test.go

@@ -0,0 +1,58 @@
+package retry
+
+import (
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestParseRetryAfter(t *testing.T) {
+	t.Run("seconds format", func(t *testing.T) {
+		resp := &http.Response{
+			Header: http.Header{"Retry-After": []string{"120"}},
+		}
+		d, ok := ParseRetryAfter(resp)
+		if !ok {
+			t.Fatal("expected ok=true")
+		}
+		if d != 120*time.Second {
+			t.Fatalf("expected 120s, got %v", d)
+		}
+	})
+
+	t.Run("empty header", func(t *testing.T) {
+		resp := &http.Response{
+			Header: make(http.Header),
+		}
+		d, ok := ParseRetryAfter(resp)
+		if ok {
+			t.Fatal("expected ok=false for empty header")
+		}
+		if d != 0 {
+			t.Fatalf("expected 0, got %v", d)
+		}
+	})
+
+	t.Run("nil response", func(t *testing.T) {
+		d, ok := ParseRetryAfter(nil)
+		if ok {
+			t.Fatal("expected ok=false for nil response")
+		}
+		if d != 0 {
+			t.Fatalf("expected 0, got %v", d)
+		}
+	})
+
+	t.Run("negative value", func(t *testing.T) {
+		resp := &http.Response{
+			Header: http.Header{"Retry-After": []string{"-5"}},
+		}
+		d, ok := ParseRetryAfter(resp)
+		if ok {
+			t.Fatal("expected ok=false for negative value")
+		}
+		if d != 0 {
+			t.Fatalf("expected 0, got %v", d)
+		}
+	})
+}