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.

從連結的 StackBlitz 重現開始，特別關注 src/main.ts，並重現對 authState() 的兩次連續 subscribe 呼叫。觀察 ApplicationRef.isStable，並將結果與應用程式啟動後預期發出的 true 進行比較；在此重現中應用程式變得穩定即表示完成。

由索引模型根據 Issue 內容生成。