Adding test for legacy checkpoint created with 2.5.6

[pl-ghost]

This is automated addition of created checkpoint with the latest 2.5.6 lightning release!

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu-guardian	success	✅

These checks are required after the changes to tests/legacy/back-compatible-versions.txt.

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Thank you for your contribution! 💜

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (19912d0) to head (b0225cf).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (19912d0) and HEAD (b0225cf). Click for more details.

HEAD has 351 uploads less than BASE

Flag	BASE (19912d0)	HEAD (b0225cf)
cpu	108	27
lightning_fabric	24	0
pytest	54	0
python3.12	12	3
python	12	3
python3.10	24	6
python3.12.7	36	9
lightning	60	15
python3.11	24	6
pytorch2.1	12	6
pytest-full	54	27
pytorch2.2.2	6	3
pytorch2.4.1	6	3
pytorch_lightning	24	12
pytorch2.7	6	3
pytorch2.8	6	3
pytorch2.5.1	6	3
pytorch2.6	6	3
pytorch2.3	6	3

Additional details and impacted files

@@ Coverage Diff @@ ## master #21338 +/- ## ========================================= - Coverage 87% 79% -8%  ========================================= Files 270 267 -3 Lines 23799 23744 -55 ========================================= - Hits 20629 18696 -1933  - Misses 3170 5048 +1878