fix: set nonzero exit code for SIGTERMException

[javierdejesusda]

What does this PR do?

Fixes #19916

SIGTERMException inherits from SystemExit but was raised without arguments, so SystemExit.code defaulted to None — which Python converts to exit code 0. This made it impossible for wrapper scripts or job schedulers (e.g. SLURM) to distinguish a SIGTERM kill from a clean exit.

The fix overrides __init__ to pass 143 (128 + signal 15) to SystemExit, following the standard Unix exit code convention for signal-terminated processes.

Before submitting

• Was this discussed/agreed via a GitHub issue? (KeyboardInterrupt raises an exception which results in a zero exit code #19916)
• Did you read the contributor guideline?
• Did you make sure your PR does only one thing?
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

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📚 Documentation preview 📚: https://pytorch-lightning--21623.org.readthedocs.build/en/21623/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (648ec13) to head (b4f0e88).
⚠️ Report is 4 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (648ec13) and HEAD (b4f0e88). Click for more details.

HEAD has 168 uploads less than BASE

Flag	BASE (648ec13)	HEAD (b4f0e88)
cpu	84	42
python	6	3
lightning_fabric	27	0
pytest	42	0
python3.12	24	12
lightning	30	15
python3.11	12	6
python3.13	18	9
python3.12.7	18	9
python3.10	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21623     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         270      267      -3     
  Lines       23895    23839     -56     
=========================================
- Hits        20675    18765   -1910     
- Misses       3220     5074   +1854