client: Prefer structuredContent in toolJSON

get_equity_historicals can return MCP StructuredContent with empty or
non-JSON TextContent. Prefer StructuredContent (marshal to RawMessage)
so paper tradey once/run can parse historicals.

#13

client/session.go

@@ -101,6 +101,13 @@ func toolJSON(res *mcp.CallToolResult) (json.RawMessage, error) {
 	if err := res.GetError(); err != nil {
 		return nil, err
 	}
+	if res.StructuredContent != nil {
+		b, err := json.Marshal(res.StructuredContent)
+		if err != nil {
+			return nil, err
+		}
+		return json.RawMessage(b), nil
+	}
 	var b []byte
 	for _, c := range res.Content {
 		t, ok := c.(*mcp.TextContent)

client/session_test.go

@@ -0,0 +1,87 @@
+package client
+
+import (
+	"encoding/json"
+	"errors"
+	"testing"
+
+	mcp "github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+func TestToolJSON(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		res  *mcp.CallToolResult
+		want string
+	}{
+		{
+			name: "structured-only",
+			res: &mcp.CallToolResult{
+				StructuredContent: map[string]any{"symbol": "MU"},
+			},
+			want: `{"symbol":"MU"}`,
+		},
+		{
+			name: "text JSON",
+			res: &mcp.CallToolResult{
+				Content: []mcp.Content{&mcp.TextContent{Text: `{"symbol":"MU"}`}},
+			},
+			want: `{"symbol":"MU"}`,
+		},
+		{
+			name: "text non-JSON + structured",
+			res: &mcp.CallToolResult{
+				StructuredContent: json.RawMessage(`{"symbol":"MU"}`),
+				Content:           []mcp.Content{&mcp.TextContent{Text: "not json"}},
+			},
+			want: `{"symbol":"MU"}`,
+		},
+		{
+			name: "empty",
+			res:  &mcp.CallToolResult{},
+			want: `{}`,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			got, err := toolJSON(tc.res)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if string(got) != tc.want {
+				t.Fatalf("got %s want %s", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestToolJSON_nilRes(t *testing.T) {
+	t.Parallel()
+	_, err := toolJSON(nil)
+	if err == nil || err.Error() != "empty tool result" {
+		t.Fatalf("%v", err)
+	}
+}
+
+func TestToolJSON_getError(t *testing.T) {
+	t.Parallel()
+	inner := errors.New("tool failed")
+	res := &mcp.CallToolResult{}
+	res.SetError(inner)
+	_, err := toolJSON(res)
+	if !errors.Is(err, inner) {
+		t.Fatalf("%v", err)
+	}
+}
+
+func TestToolJSON_nonJSONText(t *testing.T) {
+	t.Parallel()
+	_, err := toolJSON(&mcp.CallToolResult{
+		Content: []mcp.Content{&mcp.TextContent{Text: "not json"}},
+	})
+	if err == nil || err.Error() != "non-json tool result" {
+		t.Fatalf("%v", err)
+	}
+}