Code Quality: Prevent drag and drop race condition

[seer-by-sentry]

Resolved / Related Issues

To prevent extra work, all changes to the Files codebase must link to an approved issue marked as Ready to build. Please insert the issue number following the hashtag with the issue number that this Pull Request resolves.

• Closes FILES-APP-4G1

Steps used to test these changes

Stability is a top priority for Files and all changes are required to go through testing before being merged into the repo. Please include a list of steps that you used to test this PR.

1. Checked that drag and drop operations do not cause errors when performed immediately after the application starts.
2. Verified that ShellViewModel is properly initialized before drag and drop operations are handled.
3. Ensured that the application remains stable and responsive during and after drag and drop operations.

[Copilot]

Pull Request Overview

This PR fixes a race condition in drag and drop operations by adding null checks for the ShellViewModel before processing drag operations. The fix prevents errors that occur when drag and drop events are triggered before the application is fully initialized.

• Added null checks for _associatedInstance.ShellViewModel in both DragOverAsync and DropAsync methods
• Sets AcceptedOperation to None and returns early when ShellViewModel is not yet initialized
• Ensures proper completion of deferrals to maintain application stability

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