fix: parse MCP result payloads and drop place idempotency_key

Typed result structs replace empty envelopes. Equity place sends
ref_id only so live additionalProperties:false schemas accept the call.

equity/write_test.go

@@ -40,6 +40,26 @@ func TestPlaceOrder_decimalStrings(t *testing.T) {
 	if got["ref_id"] != "buy:2026-08-18:MU" {
 		t.Fatalf("ref %v", got["ref_id"])
 	}
+	if _, ok := got["idempotency_key"]; ok {
+		t.Fatalf("idempotency_key %v", got["idempotency_key"])
+	}
+}
+
+func TestPlaceOrder_ordersEnvelope(t *testing.T) {
+	t.Parallel()
+	c := equity.New(client.Func(func(ctx context.Context, name string, args map[string]any) (json.RawMessage, error) {
+		return json.RawMessage(`{"orders":[{"id":"o2","symbol":"MU","side":"buy","quantity":"3","state":"confirmed"}]}`), nil
+	}))
+	got, err := c.PlaceOrder(context.Background(), equity.PlaceOrderRequest{Symbol: "MU"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.ID != "o2" || got.Symbol != "MU" || got.Side != "buy" || got.State != "confirmed" {
+		t.Fatalf("%+v", got)
+	}
+	if !got.Qty.Equal(decimal.NewFromInt(3)) {
+		t.Fatalf("qty %s", got.Qty)
+	}
 }
 
 func TestEquity_writeArgs(t *testing.T) {
@@ -98,14 +118,13 @@ func TestEquity_writeArgs(t *testing.T) {
 			},
 			wantName: "place_equity_order",
 			wantArgs: map[string]any{
-				"account_number":  "acct",
-				"symbol":          "MU",
-				"side":            "sell",
-				"type":            "stop_market",
-				"quantity":        "3",
-				"stop_price":      "99.6",
-				"ref_id":          "sell:2026-08-18:MU",
-				"idempotency_key": "sell:2026-08-18:MU",
+				"account_number": "acct",
+				"symbol":         "MU",
+				"side":           "sell",
+				"type":           "stop_market",
+				"quantity":       "3",
+				"stop_price":     "99.6",
+				"ref_id":         "sell:2026-08-18:MU",
 			},
 		},
 		{
@@ -135,8 +154,7 @@ func TestEquity_writeArgs(t *testing.T) {
 				"tax_lots": []map[string]any{
 					{"open_lot_id": "lot-1", "quantity": "1.5"},
 				},
-				"ref_id":          "sell:lots",
-				"idempotency_key": "sell:lots",
+				"ref_id": "sell:lots",
 			},
 		},
 		{
@@ -182,6 +200,11 @@ func TestEquity_writeArgs(t *testing.T) {
 					t.Fatalf("time_in_force injected: %+v", gotArgs)
 				}
 			}
+			if tc.wantName == "place_equity_order" {
+				if _, ok := gotArgs["idempotency_key"]; ok {
+					t.Fatalf("idempotency_key on place: %+v", gotArgs)
+				}
+			}
 		})
 	}
 }