feat!: align metadata models with draft schema (#993)

crates/rmcp/tests/test_message_schema.rs

@@ -61,6 +61,60 @@ mod tests {
         );
     }
 
+    /// The three metadata definitions must expose the MCP 2026-07-28 draft
+    /// vocabulary: `MetaObject` is an open map, `RequestMetaObject` reserves
+    /// `progressToken` plus the SEP-2575 keys, and `NotificationMetaObject`
+    /// reserves `io.modelcontextprotocol/subscriptionId`. The keys the draft
+    /// marks as required stay optional because rmcp generates one schema
+    /// shared by every supported protocol version; draft-strict validation is
+    /// a runtime concern (`RequestMetaObject::missing_required_keys`).
+    #[test]
+    fn test_metadata_definitions_match_draft_schema() {
+        let settings = SchemaSettings::draft07();
+        let schema = settings
+            .into_generator()
+            .into_root_schema_for::<ClientJsonRpcMessage>();
+        let schema = serde_json::to_value(&schema).expect("Failed to serialize schema");
+        let definitions = &schema["definitions"];
+
+        assert_eq!(
+            definitions["MetaObject"],
+            serde_json::json!({
+                "description": "See [specification/draft/basic/index#general-fields] for notes on _meta usage.",
+                "type": "object",
+                "additionalProperties": true,
+            })
+        );
+
+        assert_eq!(
+            definitions["RequestMetaObject"],
+            serde_json::json!({
+                "description": "Metadata reserved by MCP on requests. Extension keys are also allowed.",
+                "type": "object",
+                "properties": {
+                    "progressToken": { "$ref": "#/definitions/ProgressToken" },
+                    "io.modelcontextprotocol/protocolVersion": { "type": "string" },
+                    "io.modelcontextprotocol/clientInfo": { "$ref": "#/definitions/Implementation" },
+                    "io.modelcontextprotocol/clientCapabilities": { "$ref": "#/definitions/ClientCapabilities" },
+                    "io.modelcontextprotocol/logLevel": { "$ref": "#/definitions/LoggingLevel" },
+                },
+                "additionalProperties": true,
+            })
+        );
+
+        assert_eq!(
+            definitions["NotificationMetaObject"],
+            serde_json::json!({
+                "description": "Metadata reserved by MCP on notifications. Extension keys are also allowed.",
+                "type": "object",
+                "properties": {
+                    "io.modelcontextprotocol/subscriptionId": { "$ref": "#/definitions/NumberOrString" },
+                },
+                "additionalProperties": true,
+            })
+        );
+    }
+
     #[test]
     fn test_server_json_rpc_message_schema() {
         let settings = SchemaSettings::draft07();