[MNT]: 3D z axis margin is handled differently than x and z #31213

Summary

The z-axis default margin is 0 for perspective projection, while x/y use 0.05. zz_dataLim and _zmargin are calibrated to produce the same empty-plot limits as x/y, but they diverge when data is present - x and y get margin but z does not. scatter has a special code path to bump _zmargin to 0.05, but this is missing from the other plotting methods. We should have everything on a common code path here with margins for all axes, but this will require regenerating a large number of baseline images.

Waiting on #30980, to ensure consistency across different scales. That PR also adds a test for equal blank-plot axis limits.

Proposed fix

No response