Functions: callable.stream batching behaviour

### Description

Hey there iOS team, coming here from React Native Firebase. We recently implemented callable streams in our package and encountered strange behaviour. In our test app we will see the chunks do come sequentially as intended but gets picked up in a batch on the app as if it came in one go. I went on to test it on here and it seemed to reproduce.

### Reproducing the issue

https://github.com/MichaelVerdon/callable-stream-batching-repro/tree/main
here is the repro

and on React Native Firebase
https://github.com/invertase/react-native-firebase/blob/main/tests/local-tests/functions/streaming-callable.tsx

### Firebase SDK Version

12.9.0

### Xcode Version

16.4

### Installation Method

Swift Package Manager

### Firebase Product(s)

Functions

### Targeted Platforms

iOS

### Relevant Log Output

```shell

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### If using Swift Package Manager, the project's Package.resolved

Expand Package.resolved snippet

```json

Replace this line with the contents of your Package.resolved.

```

### If using CocoaPods, the project's Podfile.lock

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```yml

Replace this line with the contents of your Podfile.lock!

```