fix CallToolResult json marshaling and unmarshaling: need structuredC… (#523)

* fix CallToolResult json marshaling and unmarshaling: need structuredContent
.

* golang json use float to store number type

* fix unittest: use NewMetaFromMap to create Result.Meta

* mcptest: add a test case to response stuctured content

* move after TestServerWithTool

* fix unittest: no need check meta

Author: sunfuze

mcp/tools.go

@@ -486,6 +486,11 @@ func (r CallToolResult) MarshalJSON() ([]byte, error) {
 	}
 	m["content"] = content
 
+	// Marshal StructuredContent if present
+	if r.StructuredContent != nil {
+		m["structuredContent"] = r.StructuredContent
+	}
+
 	// Marshal IsError if true
 	if r.IsError {
 		m["isError"] = r.IsError
@@ -526,6 +531,11 @@ func (r *CallToolResult) UnmarshalJSON(data []byte) error {
 		}
 	}
 
+	// Unmarshal StructuredContent if present
+	if structured, ok := raw["structuredContent"]; ok {
+		r.StructuredContent = structured
+	}
+
 	// Unmarshal IsError
 	if isError, ok := raw["isError"]; ok {
 		if isErrorBool, ok := isError.(bool); ok {