fix: reset request body before x402 retry to prevent exhausted reader

X402Transport.RoundTrip clones the request twice (first attempt + retry)
but Clone() shares the body reader without calling GetBody(). After the
first attempt consumes the body, the retry gets an exhausted reader,
causing 'ReverseProxy does an invalid Read on closed Body'.

Add bodyResetTransport that calls GetBody() before each RoundTrip,
ensuring each attempt gets a fresh reader. Pairs with the existing
bodyBufferMiddleware which guarantees GetBody is always set.

Author: bussyjd

internal/x402/buyer/proxy.go

@@ -217,7 +217,7 @@ func (p *Proxy) buildUpstreamHandler(name, remoteModel string, cfg UpstreamConfi
 			return nil
 		},
 		Transport: &x402http.X402Transport{
-			Base:     http.DefaultTransport,
+			Base:     &bodyResetTransport{base: http.DefaultTransport},
 			Signers:  []x402.Signer{signer},
 			Selector: x402.NewDefaultPaymentSelector(),
 			OnPaymentAttempt: func(event x402.PaymentEvent) {
@@ -336,6 +336,25 @@ func bodyBufferMiddleware(next http.Handler) http.Handler {
 	})
 }
 
+// bodyResetTransport resets the request body from GetBody before each RoundTrip.
+// This works around a bug in x402-go's X402Transport where req.Clone() shares
+// the body reader — after the first attempt consumes it, the retry gets an
+// exhausted reader. By resetting from GetBody (set by bodyBufferMiddleware),
+// each attempt gets a fresh reader.
+type bodyResetTransport struct {
+	base http.RoundTripper
+}
+
+func (t *bodyResetTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	if req.GetBody != nil {
+		body, err := req.GetBody()
+		if err == nil {
+			req.Body = body
+		}
+	}
+	return t.base.RoundTrip(req)
+}
+
 // handleStatus returns JSON with remaining auths and spend per upstream.
 func (p *Proxy) handleStatus(w http.ResponseWriter, r *http.Request) {
 	p.mu.RLock()