refactor(core,cli): useAlternateBuffer read from config (#20346)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/cli/src/test-utils/mockConfig.ts

@@ -156,6 +156,7 @@ export const createMockConfig = (overrides: Partial<Config> = {}): Config =>
     getExperiments: vi.fn().mockReturnValue(undefined),
     getHasAccessToPreviewModel: vi.fn().mockReturnValue(false),
     validatePathAccess: vi.fn().mockReturnValue(null),
+    getUseAlternateBuffer: vi.fn().mockReturnValue(false),
     ...overrides,
   }) as unknown as Config;