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)