Fix: Prevent Admin from demoting their own role (#9326)

[yasw175]

Description

This PR addresses Issue #9326.

I implemented a validation check in user_service.py (inside the update method) to prevent an Admin user from demoting their own role permissions. This fixes a logic gap where an admin could accidentally lock themselves out of administrative privileges.

Specifically:

• Added a check to verify if uid matches context.credentials (the user modifying themselves).
• Added a check to verify if the current user role is ADMIN.
• Raises a SyftError with the message "Admins cannot demote their own role!" if a demotion is attempted.

Fixes #9326

Affected Dependencies

None. This change only modifies internal logic in packages/syft/src/syft/service/user/user_service.py.

How has this been tested?

• Manual Logic Verification
• I verified the code path in user_service.py to ensure the conditional logic correctly targets only self-updates by Admin users.

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests