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.

Beginne damit, den TypeScript-Fehler im bereitgestellten Beispiel mit docData und DocumentReference zu reproduzieren, und verfolge dann die Typen der Firestore-Leseoperationen rund um Timestamp, FieldValue und WithFieldValue. Vergleiche die betroffenen Firebase SDK-Versionen und ermittle, an welcher Stelle die Leseergebnisse die schreiborientierte Union erhalten. Als abgeschlossen gilt die Aufgabe, wenn das Beispiel kompiliert und Timestamp-Felder toDate() ohne den FieldValue-Typ bereitstellen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.