[Core & Milky] Implement GroupFileEntity and file Segment

[Wesley-Young]

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[Copilot]

Pull Request Overview

This PR implements support for group file messages by adding a GroupFileEntity in the Core library and corresponding FileIncomingSegment in the Milky library. The implementation includes protobuf message definitions for parsing group file data and conversion logic to handle file segments.

• Added GroupFileEntity class to handle group file message parsing and representation
• Implemented FileIncomingSegment for the Milky API with JSON serialization support
• Added protobuf definitions for parsing group file metadata from message elements

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Lagrange.Core/Message/Entities/GroupFileEntity.cs	New entity class for group file messages with parsing logic
Lagrange.Core/Internal/Packets/Message/Extra.cs	Added protobuf classes for group file metadata structure
Lagrange.Core/Internal/Packets/Message/Elem.cs	Added TransElem protobuf class for element parsing
Lagrange.Milky/Entity/Segment/FileSegment.cs	New file segment classes for API representation
Lagrange.Milky/Utility/EntityConvert.Segment.cs	Added conversion from GroupFileEntity to FileIncomingSegment

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