Thanks for you contribution! But to be honest, this implemetation is far from ideal.

The plugin has permission.IPC and interact with the input method with FcitxRemoteService, which is the intended design; but the input method itself uses Broadcast which makes the whole process slow and inscure, because other applications can also register an receiver with the same name and intercept the otp code. Moreover, it's actually not a trival task to extract otp code from SMS messages, a simple regex could lead to many false-positives.

I've read through the changes and made comments accordingly. In short, this PR cannot be merged in current state and needs to be rewritten from the ground up.

Or if you're tired, maybe it's better to just use https://github.com/RikkaW/SmsCodeHelper . It still works on Android 15 with fcitx5-android. It's more secure too, because on Android 10+, only the foreground process and the input method has access to the clipboard.