fix(core): use returnDisplay for error result display (#14994)

Co-authored-by: Sehoon Shon <sshon@google.com>

packages/core/src/scheduler/tool-executor.ts

@@ -123,10 +123,15 @@ export class ToolExecutor {
           } else if (toolResult.error === undefined) {
             return await this.createSuccessResult(call, toolResult);
           } else {
+            const displayText =
+              typeof toolResult.returnDisplay === 'string'
+                ? toolResult.returnDisplay
+                : undefined;
             return this.createErrorResult(
               call,
               new Error(toolResult.error.message),
               toolResult.error.type,
+              displayText,
             );
           }
         } catch (executionError: unknown) {
@@ -271,8 +276,14 @@ export class ToolExecutor {
     call: ToolCall,
     error: Error,
     errorType?: ToolErrorType,
+    returnDisplay?: string,
   ): ErroredToolCall {
-    const response = this.createErrorResponse(call.request, error, errorType);
+    const response = this.createErrorResponse(
+      call.request,
+      error,
+      errorType,
+      returnDisplay,
+    );
     const startTime = 'startTime' in call ? call.startTime : undefined;
 
     return {
@@ -289,7 +300,9 @@ export class ToolExecutor {
     request: ToolCallRequestInfo,
     error: Error,
     errorType: ToolErrorType | undefined,
+    returnDisplay?: string,
   ): ToolCallResponseInfo {
+    const displayText = returnDisplay ?? error.message;
     return {
       callId: request.callId,
       error,
@@ -302,9 +315,9 @@ export class ToolExecutor {
           },
         },
       ],
-      resultDisplay: error.message,
+      resultDisplay: displayText,
       errorType,
-      contentLength: error.message.length,
+      contentLength: displayText.length,
     };
   }
 }