Prevent registering as arbitrary agents with system token

[qwerty287]

Prevent that an agent can register as any other agent by its ID if it has the system token.

This is no big thread, just hardening, as if an user has already the system token, he can create new agents anyway as he likes.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.40%. Comparing base (6d60849) to head (dd7b6f4).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
server/rpc/auth_server.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6283      +/-   ##
==========================================
- Coverage   32.43%   32.40%   -0.03%     
==========================================
  Files         420      420              
  Lines       28320    28326       +6     
==========================================
- Hits         9185     9180       -5     
- Misses      18286    18297      +11     
  Partials      849      849              

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