feat!: relax tool result structuredContent type (SEP-2106) (#933)

Re-applies #919 (reverted by #932): ToolResultContent.structured_content
becomes Option<Value> so non-object structured content is accepted,
matching CallToolResult and SEP-2106.

BREAKING CHANGE: ToolResultContent.structured_content changes from
Option<JsonObject> to Option<Value>.

crates/rmcp/src/model/content.rs

@@ -10,7 +10,7 @@
 // ToolUseContent/ToolResultContent are SEP-2577-deprecated; internal references are expected.
 #![expect(deprecated)]
 use serde::{Deserialize, Serialize};
-use serde_json::json;
+use serde_json::{Value, json};
 
 use super::{Annotations, Meta, resource::ResourceContents};
 
@@ -207,7 +207,7 @@ pub struct ToolResultContent {
     pub tool_use_id: String,
     pub content: Vec<ContentBlock>,
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub structured_content: Option<super::JsonObject>,
+    pub structured_content: Option<Value>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub is_error: Option<bool>,
 }

crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json

@@ -2397,13 +2397,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json

@@ -2397,13 +2397,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -3562,13 +3562,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

@@ -3562,13 +3562,7 @@
             "null"
           ]
         },
-        "structuredContent": {
-          "type": [
-            "object",
-            "null"
-          ],
-          "additionalProperties": true
-        },
+        "structuredContent": true,
         "toolUseId": {
           "type": "string"
         }

crates/rmcp/tests/test_sampling.rs

@@ -9,6 +9,7 @@ use rmcp::{
     model::*,
     service::{RequestContext, Service},
 };
+use rstest::rstest;
 
 #[tokio::test]
 async fn test_basic_sampling_message_creation() -> Result<()> {
@@ -370,6 +371,39 @@ fn test_tool_result_content_requires_content() {
     assert!(err.to_string().contains("missing field `content`"));
 }
 
+#[rstest]
+#[case::array(serde_json::json!([{ "city": "SF", "temp": 72 }, { "city": "NY", "temp": 65 }]))]
+#[case::string(serde_json::json!("sunny"))]
+#[case::integer(serde_json::json!(42))]
+#[case::float(serde_json::json!(3.14))]
+#[case::boolean(serde_json::json!(true))]
+fn tool_result_content_round_trips_non_object_structured_content(
+    #[case] structured: serde_json::Value,
+) -> Result<()> {
+    let mut tool_result = ToolResultContent::new("call_123", vec![ContentBlock::text("x")]);
+    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);
+
+    Ok(())
+}
+
+// `null` is a valid JSON value, but `Option<Value>` deserializes JSON `null`
+// as `None`, so `Some(Value::Null)` collapses to `None` on round-trip.
+#[test]
+fn tool_result_content_null_structured_content_round_trips_to_none() -> Result<()> {
+    let mut tool_result = ToolResultContent::new("call_123", vec![ContentBlock::text("x")]);
+    tool_result.structured_content = Some(serde_json::Value::Null);
+
+    let json = serde_json::to_string(&tool_result)?;
+    let deserialized: ToolResultContent = serde_json::from_str(&json)?;
+    assert_eq!(deserialized.structured_content, None);
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn test_sampling_message_with_tool_use() -> Result<()> {
     let message = SamplingMessage::assistant_tool_use(