Fix multi-file diff error handling and UI feedback

[daniel-lxs]

This PR fixes several issues with multi-file diff error handling:

• Fixed nested array structure: Resolved issue where failed diff parts were creating nested arrays, causing error handling to malfunction
• Consolidated error reporting: All diff errors are now collected and reported in a single message instead of multiple separate calls
• Fixed infinite spinner: Single file diff failures now properly update the UI to stop the loading spinner

Resolves #4664

---

Important

Fixes error handling and UI feedback for multi-file diffs, addressing nested arrays, consolidated error reporting, and UI spinner issues.

• Error Handling:
  • Fix nested array issue in applyDiff() in multi-file-search-replace.ts by flattening failParts.
  • Consolidate error reporting in applyDiffTool() in multiApplyDiffTool.ts by collecting all errors in allDiffErrors.
• UI Feedback:
  • Resolve infinite spinner issue in applyDiffTool() in multiApplyDiffTool.ts by sending a complete message for single file operations.
• Misc:
  • Remove unnecessary line in multiApplyDiffTool.ts for cleaner code.

^{This description was created by for f8bd001. You can customize this summary. It will automatically update as commits are pushed.}