fix: Adjust the logic in multiTerminalLoginChecker to prevent onlineUserNum from decreasing below zero, thereby avoiding negative values.

[FGadvancer]

🔍 What type of PR is this?

👀 What this PR does / why we need it:

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

🅰 Which issue(s) this PR fixes:

Fixes #1657

📝 Special notes for your reviewer:

🎯 Describe how to verify it

📑 Additional documentation e.g., RFC, notion, Google docs, usage docs, etc.:

[sweep-ai]

Apply Sweep Rules to your PR?

• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.
• Apply: Add docstrings to all functions and file headers.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d594d6f) 5.11% compared to head (677ee2c) 5.12%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/msggateway/n_ws_server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1658   +/-   ##
=====================================
  Coverage   5.11%   5.12%           
=====================================
  Files         42      42           
  Lines       3792    3789    -3     
=====================================
  Hits         194     194           
+ Misses      3587    3584    -3     
  Partials      11      11           

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