Fix crash in egui_extras::FileLoader after forget_image (#6995)

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

crates/egui/src/context.rs

@@ -3446,6 +3446,7 @@ impl Context {
     /// Release all memory and textures related to the given image URI.
     ///
     /// If you attempt to load the image again, it will be reloaded from scratch.
+    /// Also this cancels any ongoing loading of the image.
     pub fn forget_image(&self, uri: &str) {
         use load::BytesLoader as _;