fix: expose unload() JSI binding for TextToImageModule

TextToImage.unload() exists in C++ but was never registered as a JSI
host function, causing `TypeError: this.nativeModule.unload is not a
function` when calling BaseModule.delete() from JS.

Every other composite model (OCR, VerticalOCR, Kokoro, LLM) already
registers unload in its `if constexpr` block — this adds the same
one-liner for TextToImage.

Author: yocontra

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

@@ -148,6 +148,8 @@ template <typename Model> class ModelHostObject : public JsiHostObject {
     }
 
     if constexpr (meta::SameAs<Model, models::text_to_image::TextToImage>) {
+      addFunctions(
+          JSI_EXPORT_FUNCTION(ModelHostObject<Model>, unload, "unload"));
       addFunctions(JSI_EXPORT_FUNCTION(
           ModelHostObject<Model>, synchronousHostFunction<&Model::interrupt>,
           "interrupt"));