fix: Prevent concurrent FileUpload init() calls

[People-Sea]

Description

#19154 introduced a ResizeObserver to handle visibility changes, but since init() is async, the observer can fire during await this.getFiles() and trigger a second init() before the first completes. This race condition corrupts FilePond's internal state, causing a console error (though the component still works).

Added an isInitializing flag to guard against concurrent init() calls.

Visual changes

before

before.mov

after

after.mov

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.