test: Fix flaky full-stack tests by mocking service ports

[okurz]

Motivation:
Multiple full-stack tests were failing intermittently when run in parallel
due to "Address already in use" errors. They all defaulted to the same
base port (9526), causing conflicts in concurrent test environments.

Design Choices:

• Restored mock_service_ports in OpenQA::Test::Utils which was previously
  removed as dead code in 8d7b9a5. It mocks OpenQA::Utils::service_port
  via Test::MockModule to allocate independent free ports per service using
  Mojo::IOLoop::Server->generate_port, which asks the OS for a truly free
  port instead of blindly guessing.
• Applied mock_service_ports to t/05-scheduler-full.t, t/25-cache-service.t,
  t/33-developer_mode.t, t/43-scheduling-and-worker-scalability.t, and
  t/full-stack.t, replacing the prior setup_random_base_port approach.
• Each service gets an independently chosen free port (not a fixed-offset
  block), eliminating the risk that one occupied port breaks all services.
• Removed duplicate randomization logic and redundant comments across
  multiple test files.
• Cleaned up BEGIN blocks and imports in full-stack tests.

Benefits:

• Guarantees free ports via OS-level allocation, eliminating race conditions
  that setup_random_base_port could not prevent.
• Eliminates port conflicts during parallel test execution.
• Improves test stability and reliability for CI/CD.
• Centralizes logic for easier future maintenance and consistent
  port selection strategy.
• Cleaner and more maintainable test code.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.96%. Comparing base (91af0a8) to head (a92fc66).
⚠️ Report is 87 commits behind head on master.

Files with missing lines	Patch %	Lines
t/lib/OpenQA/Test/Utils.pm	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7244      +/-   ##
==========================================
- Coverage   99.97%   99.96%   -0.01%     
==========================================
  Files         419      419              
  Lines       44141    44155      +14     
==========================================
+ Hits        44128    44138      +10     
- Misses         13       17       +4     

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[perlpunk]

Now I'm wondering why mock_service_ports was removed: 8d7b9a5
I guess the according comment block was overlooked.

[okurz]

huh. That's why this comment block was a bit dangling :D

[okurz]

replaced all the randomization by using re-introduced mock_service_ports