Feature/2fa plugin for client user

[8tuso]

Summary

This PR introduces significant updates to the core 2FA authentication logic within the plugin.

The changes are designed to align with and extend the new 2FA support introduced in the main osTicket repository.

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Changes

• Refactored core 2FA authentication flow within the plugin
• Improved integration with the updated client authentication process
• Adjusted validation and verification handling
• Ensured compatibility with the updated core implementation

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Dependencies

This PR must be used together with the corresponding core changes:

• Feature/2fa support for client user osTicket#6953

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Testing

• Verified full 2FA flow with updated core changes
• Tested authentication success and failure scenarios
• Confirmed correct interaction between plugin and core logic

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Notes

• These changes are tightly coupled with the core PR and are not intended to be used independently
• Feedback and detailed review are highly appreciated, especially around authentication flow and edge cases