fix: prevent duplicate update operations in queue (#4131)

[Janek91]

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• Have you checked that there aren't other open pull requests for the same changes?
• Have you tested that the committed code can be executed without errors?
• This PR is not composed of garbage changes used to farm GitHub activity to enter potential Crypto AirDrops.
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Description

This PR adds duplicate detection logic to prevent the same package update operation from being queued multiple times during unattended updates.

Problem

When unattended auto-updates trigger while the user is away from their PC (particularly when UAC prompts are waiting for user interaction), the UpdateAll() method runs repeatedly and creates dozens of duplicate operations for the same package. This happens because:

1. Auto-updates call UpdateAll() periodically (every 60 minutes on success, 10 minutes on failure)
2. When UAC prompts are waiting, operations are created but package tags aren't set immediately
3. The next UpdateAll() call doesn't detect the existing operation and creates duplicates
4. Users return to find dozens of duplicate operations and UAC prompts to cancel

Solution

Added duplicate detection in the Update() method (src/UniGetUI/AppOperationHelper.cs) that:

• Checks the operation list before creating a new UpdatePackageOperation
• Uses GetHash() to compare packages by Manager, Source, and Package ID
• Only checks operations with status InQueue or Running
• Returns the existing operation if a duplicate is detected, preventing queue pollution
• Logs an informational message when duplicates are skipped

Changes

• Modified: src/UniGetUI/AppOperationHelper.cs - Added 14 lines of duplicate detection logic

Testing

• ✅ Code compiles without errors
• ✅ No linter errors introduced
• ✅ Uses existing infrastructure (operation list and package hash comparison)
• ✅ Minimal, focused change with clear logging

Expected Impact

After this fix:

• Users will see only one update operation per package
• Only one UAC prompt per package update
• No more dozens of duplicate operations to cancel
• Cleaner operation queue even when away during auto-updates

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Closes #4131

[Janek91]

Thanks for the feedback! You're absolutely right - I've moved the duplicate detection logic to only apply during automatic updates.
I've also added tests in UniGetUI.PackageEngine.Serializable.Tests that verify Package.GetHash() works correctly for duplicate detection
The duplicate detection now only triggers during unattended update scenarios (like the issue #4131 describes), while manual user actions remain unrestricted.