Remove Subscribe property from User class

[Copilot]

This PR removes the Subscribe property from the User class and updates all associated tests to reflect this change.

Changes

Model Changes

• Removed the Subscribe property declaration from the User class in Gorse.NET/Models/User.cs

Test Updates

Updated TestUsersAPI.cs to remove all references to the Subscribe property in the following test methods:

• TestUserLifecycle() - Removed Subscribe initialization and assignment
• TestUserLifecycleAsync() - Removed Subscribe assignment
• TestUsersBatch() - Removed Subscribe initialization for both test users
• TestUsersBatchAsync() - Removed Subscribe initialization for both test users

Verification

• ✅ Solution builds successfully in both Debug and Release configurations
• ✅ No compilation errors or warnings related to the changes
• ✅ All Subscribe references have been removed from the codebase

Fixes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>remove Subscribe from User</issue_title>
<issue_description>Remove Subscribe struct from User and its tests.
</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #20

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