Use RMCP for StreamableHTTP OAuth support (#3845)

crates/goose-cli/src/scenario_tests/mock_client.rs

@@ -96,8 +96,9 @@ impl McpClientTrait for MockClient {
         if let Some(handler) = self.handlers.get(name) {
             match handler(&arguments) {
                 Ok(content) => Ok(CallToolResult {
-                    content,
+                    content: Some(content),
                     is_error: None,
+                    structured_content: None,
                 }),
                 Err(e) => Err(Error::UnexpectedResponse),
             }