fix(core): prevent omission placeholder deletions in replace/write_file (#19870)

Co-authored-by: Bryan Morgan <bryanmorgan@google.com>

packages/core/src/tools/write-file.ts

@@ -47,6 +47,7 @@ import type { MessageBus } from '../confirmation-bus/message-bus.js';
 import { debugLogger } from '../utils/debugLogger.js';
 import { WRITE_FILE_DEFINITION } from './definitions/coreTools.js';
 import { resolveToolDeclaration } from './definitions/resolver.js';
+import { detectOmissionPlaceholders } from './omissionPlaceholderDetector.js';
 
 /**
  * Parameters for the WriteFile tool
@@ -486,6 +487,11 @@ export class WriteFileTool
       }`;
     }
 
+    const omissionPlaceholders = detectOmissionPlaceholders(params.content);
+    if (omissionPlaceholders.length > 0) {
+      return "`content` contains an omission placeholder (for example 'rest of methods ...'). Provide complete file content.";
+    }
+
     return null;
   }