`isolatedDeclarations` quick-fix cannot consistently use auto-imports

@DanielRosenwasser đang làm issue này rồi.

Từ ngày 23/6/2026.

• #4412 của @copilot-swe-agent — đã đóng, không merge

```ts
// @module: preserve
// @isolatedDeclarations: true

// @Filename: types.ts
export type Thing = {};

// @Filename: funcs.ts
import type { Thing } from "./types";

export function makeThing(): Thing {
throw {};
}

// @Filename: exporter.ts
import { makeThing } from "./funcs";

export let thing = makeThing();
```

The test has `isolatedDeclarations` set to true, so `export let thing` needs an annotation.

Ordinarily, we have a best-effort import adder for `Thing` which works if `Thing` is available through an existing import; but because `exporter.ts` doesn't import `types.ts`, it doesn't work in this case.