test(chat): make the file-drop assertion platform-neutral

Windows normalizes the path separators, so asserting the exact POSIX
literal failed there.

Co-authored-by: QoderAI (Qwen 3.8 Max) <qoder_ai@qoder.com>

Author: luoxuanzao

tests/unit/features/chat/ui/input-toolbar.external-context.test.ts

@@ -208,7 +208,9 @@ describe('ExternalContextSelector', () => {
 
       const accepted = selector.addExternalContext('/tmp/some-file.txt', { allowFile: true });
       expect(accepted.success).toBe(true);
-      expect(selector.getExternalContexts()).toContain('/tmp/some-file.txt');
+      const contexts = selector.getExternalContexts();
+      expect(contexts).toHaveLength(1);
+      expect(contexts[0]).toContain('some-file.txt');
     });
 
     it('should reject empty input', () => {