[Core] Fix upload_private_file

[kengwang]

Fix #70

Maybe not that elegant

[Copilot]

Pull Request Overview

This PR fixes issue #70 related to file upload functionality by correcting the MD5 checksum used in private file uploads. The main issue was that the protocol expects the MD5 hash of the first ~10MB of the file in certain fields, not the full file MD5.

Key changes:

• Refactored MD5 checksum calculation to be performed before creating the upload request
• Fixed NotOnlineFile.FileMd5 to use the correct 10MB MD5 checksum instead of the full file MD5
• Added File10MMd5 parameter to FileUploadEventReq to pass the 10MB checksum explicitly

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
Lagrange.Core/Message/MessagePacker.cs	Fixed to use File10MMd5 instead of FileMd5 for the protocol message field
Lagrange.Core/Internal/Services/System/FileUploadService.cs	Removed duplicate MD5 calculation logic, now uses the pre-calculated value from the request
Lagrange.Core/Internal/Logic/OperationLogic.cs	Moved 10MB MD5 calculation before request creation; applied same fix to both SendFriendFile and SendGroupFile methods
Lagrange.Core/Internal/Events/System/FileUploadEvent.cs	Added md510m parameter and File10MMd5 property to the request event

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