Handle Docker multi-protocol port bindings when creating challenge containers

[Copilot]

• Review the new PR comments and identify the exact requested follow-up changes
• Update DockerManager to use is [] for the non-null empty binding check
• Remove the DockerManager unit test file per review feedback
• Re-run backend unit tests to validate the follow-up changes
• Review the final diff for patch cleanliness

[codecov]

Codecov Report

❌ Patch coverage is 47.36842% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.31%. Comparing base (4ba7b62) to head (9ac70ee).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../GZCTF/Services/Container/Manager/DockerManager.cs	47.36%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #576      +/-   ##
===========================================
+ Coverage    59.11%   59.31%   +0.19%     
===========================================
  Files          184      184              
  Lines         9371     9386      +15     
  Branches      1137     1141       +4     
===========================================
+ Hits          5540     5567      +27     
+ Misses        3409     3395      -14     
- Partials       422      424       +2     

Files with missing lines	Coverage Δ
.../GZCTF/Services/Container/Manager/DockerManager.cs	52.88% <47.36%> (-1.01%)	⬇️

... and 4 files with indirect coverage changes

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