fix(kokoro): register streamStop on JSI host object (#951)

## Summary

`Kokoro::streamStop()` exists in C++ (`Kokoro.cpp:205`) and is called
from the JS `TextToSpeechModule.streamStop()` via
`this.nativeModule.streamStop()`, but it's never registered on the JSI
host object in `ModelHostObject.h`.

This causes a runtime error when stopping TTS streaming:

```
TypeError: this.nativeModule.streamStop is not a function (it is undefined)
```

The fix registers `streamStop` as a `synchronousHostFunction` in the
Kokoro `if constexpr` block, matching the existing pattern used for
`SpeechToText`.

## Changes

- `ModelHostObject.h`: Add `streamStop` JSI registration for Kokoro
model type

Author: yocontra

packages/react-native-executorch/common/rnexecutorch/host_objects/ModelHostObject.h

@@ -171,6 +171,10 @@ template <typename Model> class ModelHostObject : public JsiHostObject {
       addFunctions(JSI_EXPORT_FUNCTION(ModelHostObject<Model>,
                                        promiseHostFunction<&Model::stream>,
                                        "stream"));
+      addFunctions(JSI_EXPORT_FUNCTION(
+          ModelHostObject<Model>,
+          synchronousHostFunction<&Model::streamStop>,
+          "streamStop"));
       addFunctions(JSI_EXPORT_FUNCTION(
           ModelHostObject<Model>,
           promiseHostFunction<&Model::generateFromPhonemes>,