Fix EDT violation in course additional files check

[meanmail]

Overview

File I/O operations are being performed on the EDT (Event Dispatch Thread) during course update checks, which can freeze the UI.

Location

File: intellij-plugin/hs-core/src/org/hyperskill/academy/learning/stepik/hyperskill/update/HyperskillCourseUpdater.kt
Function: needUpdateCourseAdditionalFiles (lines 313-334)

Problem

The function file.loadEncodedContent() is wrapped in runWriteAction inside invokeAndWaitIfNeeded, which blocks the EDT. This is called in a loop during course update checks and can cause UI freezes.

val localFileContent = invokeAndWaitIfNeeded {
    runWriteAction {
        file.loadEncodedContent()
    }
}

What needs to be fixed

• Move file reading operations to a background thread using ReadAction.compute() or ReadAction.nonBlocking()
• Keep only actual write operations in write actions
• The function is called in a loop, so the performance impact is significant
• Ensure proper threading model: IO operations should not block EDT

Tests understanding of

• EDT vs background thread operations
• ReadAction / WriteAction proper usage
• invokeAndWaitIfNeeded gotchas
• File system operations threading requirements
• Smart mode awareness

Validation

• Run IDE with the plugin and trigger course updates
• Ensure no UI freezes occur during course synchronization
• Add/modify tests to verify the fix

Estimated time

3-5 hours