Type Error: toDate() not found on Timestamp fields in 21.0.0-rc.0

## Issue

After upgrading to `@angular/fire@21.0.0-rc.0`, TypeScript compilation fails when calling `.toDate()` on `Timestamp` fields from Firestore documents.

## Error

```
TS2339: Property 'toDate' does not exist on type 'FieldValue | WithFieldValue'.
Property 'toDate' does not exist on type 'FieldValue'.
```

## Reproduction

```typescript
import { doc, docData, Timestamp } from '@angular/fire/firestore';

interface Member {
createdAt: Timestamp;
}

const member$ = docData(doc(firestore, 'members/123') as DocumentReference);

member$.subscribe((member) => {
// ERROR: Property 'toDate' does not exist
const date = member?.createdAt.toDate();
});
```

## Environment

- @angular/fire: 21.0.0-rc.0
- @angular/core: 21.0.1
- firebase: 12.6.0 (from 11.8.0)
- TypeScript: 5.9.3

## Analysis

`Timestamp` fields are being typed as `FieldValue | WithFieldValue` instead of just `Timestamp`. The `WithFieldValue` type (meant for writes) is incorrectly being applied to read operations, causing TypeScript to think the field could be `FieldValue`, which doesn't have `.toDate()`.

This appears to be a regression from the Firebase SDK v11→v12 upgrade in this RC.

まず、提供された docData と DocumentReference の例で TypeScript エラーを再現し、次に Timestamp、FieldValue、WithFieldValue 周辺の Firestore 読み取り操作の型を追跡します。影響を受ける Firebase SDK のバージョンを比較し、読み取り結果が書き込み用のユニオンを取得する箇所を特定します。例がコンパイルでき、Timestamp フィールドが FieldValue 型なしで toDate() を公開すれば完了です。

索引モデルが issue の本文から書いたものです。