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/backoff.go

@@ -0,0 +1,64 @@
+package retry
+
+import (
+	"math/rand/v2"
+	"time"
+)
+
+// Backoff computes the delay before the next retry attempt.
+type Backoff interface {
+	// Delay returns the wait duration for the given attempt number (zero-based).
+	Delay(attempt int) time.Duration
+}
+
+// ExponentialBackoff returns a Backoff that doubles the delay on each attempt.
+// The delay is calculated as base * 2^attempt, capped at max. When withJitter
+// is true, a random duration in [0, delay*0.5) is added.
+func ExponentialBackoff(base, max time.Duration, withJitter bool) Backoff {
+	return &exponentialBackoff{
+		base:       base,
+		max:        max,
+		withJitter: withJitter,
+	}
+}
+
+// ConstantBackoff returns a Backoff that always returns the same delay.
+func ConstantBackoff(d time.Duration) Backoff {
+	return constantBackoff{delay: d}
+}
+
+type exponentialBackoff struct {
+	base       time.Duration
+	max        time.Duration
+	withJitter bool
+}
+
+func (b *exponentialBackoff) Delay(attempt int) time.Duration {
+	delay := b.base
+	for range attempt {
+		delay *= 2
+		if delay >= b.max {
+			delay = b.max
+			break
+		}
+	}
+
+	if b.withJitter {
+		jitter := time.Duration(rand.Int64N(int64(delay / 2)))
+		delay += jitter
+	}
+
+	if delay > b.max {
+		delay = b.max
+	}
+
+	return delay
+}
+
+type constantBackoff struct {
+	delay time.Duration
+}
+
+func (b constantBackoff) Delay(_ int) time.Duration {
+	return b.delay
+}