test(api): align tests with new envelope codes

Update API handler tests to expect numeric `code`, `success` messages, and
new envelope fields (`trace_id`, `details`), matching recent response
envelope changes.

internal/api/response_test.go

@@ -7,9 +7,11 @@ import (
 )
 
 type testEnvelope struct {
-	Code    string          `json:"code"`
+	Code    int             `json:"code"`
 	Data    json.RawMessage `json:"data"`
 	Message string          `json:"message"`
+	TraceID string          `json:"trace_id"`
+	Details any             `json:"details,omitempty"`
 }
 
 func decodeEnvelope(t *testing.T, rr *httptest.ResponseRecorder, out any) testEnvelope {