feat(fabric): introduce process-safe port management

[littlebullGit]

What does this PR do?

Addresses comment: #21309 (comment)

Summary

• replace the thread-only port manager with a process-safe implementation backed by a shared JSON state file and platform-aware file locks
• add reusable [FileLock] and [PortState] utilities with comprehensive unit and multiprocessing regression coverage
• document the design in Process-Safe Port Manager design doc
• Potential supersedes fix: update port manager to use multiprocessing lock for process safety #21311

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📚 Documentation preview 📚: https://pytorch-lightning--21313.org.readthedocs.build/en/21313/

[codecov]

Codecov Report

❌ Patch coverage is 92.93478% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 87%. Comparing base (a883890) to head (3612cc4).

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #21313    +/-   ##
========================================
- Coverage      87%      87%    -1%     
========================================
  Files         270      272     +2     
  Lines       23822    24150   +328     
========================================
+ Hits        20795    20950   +155     
- Misses       3027     3200   +173     

[SkafteNicki]

@littlebullGit PR is looking good, can you please check the failing check and see if it just a random failure (it seemed to work before merging master)

[littlebullGit]

@littlebullGit PR is looking good, can you please check the failing check and see if it just a random failure (it seemed to work before merging master)

@SkafteNicki , the error seems not related. It failed to start a server within 20 sec. Maybe just try again.
E Exception: The server didn't start within 20 seconds.
../../.venv/lib/python3.9/site-packages/pytorch_lightning/serve/servable_module_validator.py:111: Exception
FAILED serve/test_servable_module_validator.py::test_servable_module_validator - Exception: The server didn't start within 20 seconds.

FAILED serve/test_servable_module_validator.py::test_servable_module_validator - Exception: The server didn't start within 20 seconds.
= 1 failed, 3139 passed, 564 skipped, 9 xfailed, 5329 warnings, 3 rerun in 1101.09s (0:18:21) =
Error: Process completed with exit code 1.