New agora_rtc_engine 6.6.2 is incompatible with ffi ^2.0.0

### Version of the agora_rtc_engine

6.6.2

### Platforms affected

- [x] Android
- [x] iOS
- [x] macOS
- [x] Windows
- [ ] Web

### Steps to reproduce

1. Use a plugin that requires ffi: ^2.0.0
2. Try and add agora_rtc_engine: 6.6.2 to your project

### Expected results

agora_rtc_engine is able to use ffi: ^2.0.0

### Actual results

agora_rtc_engine requires ffi: ">=1.2.0 <2.0.0" and is incompatible with many major dart packages due to this.

Previously agora_rtc_engine was ffi: >=1.2.0 but this changes with the latest 6.6.2 release.

### Code sample

Code sample

```dart
dependencies:
agora_rtc_engine: 6.6.2
cryptography: ^2.7.0
```

I think this is a major issue as this is just a subset of the packages my app uses that all require >2.0.0 ffi

- cryptography 2.9.0
- device_info_plus 11.3.0
- sodium 3.4.6
- file_picker 10.3.10
- flutter_secure_storage 9.0.0
- package_info_plus 4.2.0
- path_provider 2.1.5
- And more

### Screenshots or Video

_No response_

### Logs

Logs

```console
example % flutter pub get
Resolving dependencies...
Because cryptography >=2.7.0 depends on ffi ^2.1.0 and agora_rtc_engine >=6.6.2-rc.1 depends on ffi ^1.1.2,
cryptography >=2.7.0 is incompatible with agora_rtc_engine >=6.6.2-rc.1.
So, because agora_rtc_engine_example depends on both agora_rtc_engine 6.6.2 and cryptography ^2.9.0, version
solving failed.

You can try one of the following suggestions to make the pubspec resolve:
* Consider downgrading your constraint on cryptography: flutter pub add cryptography:^2.6.1
* Consider downgrading your constraint on agora_rtc_engine: flutter pub add agora_rtc_engine:^6.5.3
Failed to update packages.
example %
```

### Flutter Doctor output

Doctor output

```console
example % flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.38.10, on macOS 26.3 25D125 darwin-arm64, locale en-US)
[✓] Android toolchain - develop for Android devices (Android SDK version 36.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 26.2)
[✓] Chrome - develop for the web
[✓] Connected device (3 available)
[✓] Network resources

• No issues found!
example %
```