fix: Assertion in egui_extras::FileLoader::load

[bircni]

This pull request modifies the BytesLoader implementation for FileLoader in crates/egui_extras/src/loaders/file_loader.rs to improve thread safety and handle unexpected states more gracefully.

Changes to thread safety and state handling:

• Updated the cache logic to check if the uri exists in the cache before inserting the result. If the uri is not found, a log message is added to indicate the loading was canceled. This change prevents overwriting cache entries unexpectedly.

• Closes https://github.com/emilk/egui/issues/6755

• [x] I have followed the instructions in the PR template

[github-actions[bot]]

Preview available at https://egui-pr-preview.github.io/pr/6995-patch6755 Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[bircni]

@emilk done Sorry completely forgot about this

[bircni]

@emilk there seems to be no Occupied Poll only has Ready and Pending

[bircni]

okay late night coding is bad haha oops

[emilk]

Thanks!