[Bug] Duplicating data message

### [REQUIRED] Please fill in the following fields:

* Pre-built SDK from the [website](https://firebase.google.com/download/cpp) or open-source from this repo: _both__
* Firebase C++ SDK version: _8.6.0____
* Problematic Firebase Component: _messaging____ (Auth, Database, etc.)
* Other Firebase Components in use: _____ (Auth, Database, etc.)
* Platform you are using the C++ SDK on: __Mac___ (Mac, Windows, or Linux)
* Platform you are targeting: __ios___ (iOS, Android, and/or desktop)

### [REQUIRED] Please describe the issue here:

When sending a message like, a second notification comes
{
"data": {
"str1": "str1",
"str2": "str2"
},
"content_available": true
}

#### Steps to reproduce:
Send message with:
"content_available": true

Have you been able to reproduce this issue with just the [Firebase C++ quickstarts](https://github.com/firebase/quickstart-cpp) ?
Yes

What's the issue repro rate? (eg 100%, 1/5 etc)
100%

What happened? How can we make the problem occur?
This could be a description, log/console output, etc.

If you have a downloadable sample project that reproduces the bug you're reporting, you will
likely receive a faster response on your issue.

#### Relevant Code:
curl --location --request POST 'https://fcm.googleapis.com/fcm/send' \
--header 'Authorization: key=....' \
--header 'Content-Type: application/json' \
--data-raw '{
"to" : "...",
"content_available": true,
"data" : {
"key1" : "value1",
"key2": "value2",
"key3": "value3"
}
}'

Start by reproducing the issue with the Firebase C++ quickstarts on iOS, using the provided curl request with content_available and data fields. Compare the observed notifications with the expected single-notification behavior and collect logs or a minimal sample. Done means the duplicate notification behavior is explained and corrected, with regression coverage where the affected messaging path is tested.

Rédigé par le modèle d'indexation à partir du texte de l'issue.