Arm the shared cooldown for rate limits tunneled through HTTP 200

Azure is the only zero-data-retention route for the gpt-5.6 family, so
its capacity 429s arrive frequently and OpenRouter forwards them inside
an HTTP 200 envelope. Those bypassed the rate controller entirely: a
paced run kept sending a request every three seconds into a throttled
endpoint, failing row by row. An in-envelope 429 now records the same
escalating cooldown as a transport 429, so later acquisitions fail fast
until the deadline passes.

internal/classification/client_test.go

@@ -11,6 +11,7 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+	"time"
 
 	"finance-duck/internal/domain"
 	"finance-duck/internal/ratelimit"
@@ -358,6 +359,30 @@ func TestMalformedEnvelopesRejected(t *testing.T) {
 	}
 }
 
+// An upstream rate limit tunneled inside an HTTP 200 envelope must arm the
+// shared cooldown like a transport 429: the next classification fails fast
+// instead of pacing another request into a throttled endpoint.
+func TestEnvelope429ArmsSharedCooldown(t *testing.T) {
+	f, d := fixture()
+	calls := 0
+	c := mockClient(t, func(w http.ResponseWriter, r *http.Request) {
+		calls++
+		_, _ = io.WriteString(w, `{"error":{"code":429,"message":"private"},"choices":[]}`)
+	})
+	c.rate.Store(&ratelimit.Controller{InitialBackoff: time.Minute})
+	_, err := c.Classify(context.Background(), f, d, true)
+	var limit *ratelimit.RateLimitError
+	if err == nil || !errors.As(err, &limit) || strings.Contains(err.Error(), "private") {
+		t.Fatalf("envelope 429 not reported as a rate limit: %v", err)
+	}
+	if _, err = c.Classify(context.Background(), f, d, true); err == nil || !errors.As(err, &limit) {
+		t.Fatalf("cooldown not armed: %v", err)
+	}
+	if calls != 1 {
+		t.Fatalf("throttled endpoint was contacted again: %d calls", calls)
+	}
+}
+
 type failingTransport struct{}
 
 func (failingTransport) RoundTrip(*http.Request) (*http.Response, error) {