Looks like the BaseExtension was removed with the latest AGP 9 release:
The extension wasn't removed, but it's not created when the Hilt plugin checks for it. Unsure where the issue lies for this, but leaving this issue open.
Caused by: java.lang.IllegalStateException: Android BaseExtension not found.
at dagger.hilt.android.plugin.HiltGradlePlugin.configureCompileClasspath(HiltGradlePlugin.kt:139)
at dagger.hilt.android.plugin.HiltGradlePlugin.configureHilt(HiltGradlePlugin.kt:109)
at dagger.hilt.android.plugin.HiltGradlePlugin.apply$lambda$0(HiltGradlePlugin.kt:79)
at dagger.hilt.android.plugin.HiltGradlePlugin.apply$lambda$1(HiltGradlePlugin.kt:77)