Preserve log formatter when setting log scales

[cvanelteren]

Summary

• Reapply the log formatter when log scales are set and rc["formatter.log"] is enabled.
• Ensure ax.format(yscale="log") and ax.set_yscale("log") keep the log formatter.
• Add a focused test for the new behavior.

Testing

• pytest ultraplot/tests/test_plot.py::test_format_log_scale_preserves_log_formatter

Closes #436

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/axes/cartesian.py	92.00%	1 Missing and 1 partial ⚠️

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