Expose logger logging mode and stabilize rank-zero logging

[MagellaX]

What does this PR do?

Expose the current logging mode ("step" or "epoch") to loggers so they can adapt behavior based on the caller context. The connector now sets this mode around log_metrics, and a test covers it.

Additionally, this makes rank-zero logging consistently use the PyTorch logger (fixes debug log capture in tests) and skips symlink-dependent tests on platforms that don’t support symlinks. The model registry doctest was updated to be OS-path-agnostic.

Fixes #20502

Dependencies: None

Breaking changes: None

Tests:

• python -m pytest tests/tests_pytorch/trainer/connectors/test_logger_connector.py -q
• python -m pytest tests/tests_pytorch/loops/test_training_loop.py::test_fit_loop_done_log_messages -q
• python -m pytest tests/tests_pytorch/loops/test_training_loop.py::test_should_stop_early_stopping_conditions_met -q
• python -m pytest tests/tests_pytorch/checkpointing/test_model_checkpoint.py -q
• python -m pytest tests/tests_pytorch/loggers/test_tensorboard.py -q
• python -m pytest src/lightning/pytorch tests/tests_pytorch -q (timed out after 30 min)

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📚 Documentation preview 📚: https://pytorch-lightning--21526.org.readthedocs.build/en/21526/

[MagellaX]

can anybody review this and merge it!!!?