Two consecutive `subscribe` calls on the Observable created by `authState()` cause Angular app to never become stable.

### Version info

**Angular:** 18.0.3

**Firebase:** 10.12.2

**AngularFire:** 18.0.1

### How to reproduce these conditions

**Failing test unit, Stackblitz demonstrating the problem**

Stackblitz example with the problem: https://stackblitz.com/edit/stackblitz-starters-zfidzt?file=src%2Fmain.ts

**Steps to set up and reproduce**

- Open Stackblitz reproduction
- Check the console, and see that the `ApplicationRef.isStable` never emits true

### Debug output

**Errors in the JavaScript console**

message: `isStable: true` is expected, but it never comes

### Expected behavior

`ApplicationRef.isStable` emits `true` after application start.
### Actual behavior

`ApplicationRef.isStable` never emits `true`, if two consecutive `subscribe` calls on the Observable created by `authState()` are made.

Inizia con la riproduzione StackBlitz collegata, soprattutto con src/main.ts, e riproduci le due chiamate consecutive a subscribe su authState(). Osserva ApplicationRef.isStable e confronta il risultato con l’emissione true prevista dopo l’avvio dell’applicazione; il lavoro è completato quando l’applicazione diventa stabile in questa riproduzione.

Scritto dal modello di indicizzazione a partire dal testo della issue.