API key persistence setting is global, not per fetcher

### JabRef version Latest development branch build (please note build date below) ### Operating system Windows ### Details on version and operating system Windows 11 25H2 ### Checked with the [latest development build](https://builds.jabref.org/main/) (copy version output from About dialog) - [x] I made a backup of my libraries before testing the latest development version. - [x] I have tested the latest development version and the problem persists ### Steps to reproduce the behaviour - Configure fetcher A as non-persistent, then B as persistent → the flag ends up true (B's dialog was last) → at save time, `storeFetcherKeys()` writes all custom keys to the keyring, since it only checks the one global flag, not each key individually → both end up persisted. - Configure A as persistent, then B as non-persistent → the flag ends up false → the app wipes every key from the keyring, not just B's → both end up non-persistent. _Originally posted by @kerollosy in https://github.com/JabRef/jabref/issues/16519#issuecomment-5264001107_