feat!: relax tool result structuredContent type (#919)

crates/rmcp/tests/test_sampling.rs

@@ -370,6 +370,38 @@ fn test_tool_result_content_requires_content() {
     assert!(err.to_string().contains("missing field `content`"));
 }
 
+#[tokio::test]
+async fn test_tool_result_content_with_array_structured_content() -> Result<()> {
+    let structured =
+        serde_json::json!([{ "city": "SF", "temp": 72 }, { "city": "NY", "temp": 65 }]);
+    let mut tool_result = ToolResultContent::new("call_123", vec![ContentBlock::text("forecast")]);
+    tool_result.structured_content = Some(structured);
+
+    let json = serde_json::to_string(&tool_result)?;
+    let deserialized: ToolResultContent = serde_json::from_str(&json)?;
+    assert_eq!(tool_result, deserialized);
+    assert!(deserialized.structured_content.unwrap().is_array());
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn test_tool_result_content_with_primitive_structured_content() -> Result<()> {
+    let structured = serde_json::json!(42);
+    let mut tool_result = ToolResultContent::new("call_123", vec![ContentBlock::text("count")]);
+    tool_result.structured_content = Some(structured);
+
+    let json = serde_json::to_string(&tool_result)?;
+    let deserialized: ToolResultContent = serde_json::from_str(&json)?;
+    assert_eq!(tool_result, deserialized);
+    assert!(matches!(
+        deserialized.structured_content,
+        Some(serde_json::Value::Number(_))
+    ));
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn test_sampling_message_with_tool_use() -> Result<()> {
     let message = SamplingMessage::assistant_tool_use(