Problem

When saving a 3D figure with fig.savefig("out.png", bbox_inches='tight'),
axis labels (zlabel, xlabel, ylabel) are clipped because
Axes3D.get_tightbbox unconditionally calls
_get_tightbbox_for_layout_only for each 3D axis, which always passes
for_layout_only=True and therefore explicitly excludes labels from
the bounding box.

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(projection='3d')
ax.set_xlabel('X axis')
ax.set_zlabel('Z axis')
ax.scatter([1], [1], [1])
fig.savefig('out.png', bbox_inches='tight')  # labels get clipped

Fixes #28117.

Fix

In Axes3D.get_tightbbox, differentiate between the two call paths:

• for_layout_only=True (layout engine): continue using
  _get_tightbbox_for_layout_only so layout calculations are unaffected
• for_layout_only=False (savefig tight path): call
  axis.get_tightbbox(renderer, for_layout_only=False) directly so that
  labels are included in the bounding box

Tests

Added test_axes3d_tightbbox_includes_axis_labels which:

• Verifies the full bbox is at least as large as the layout-only bbox
  (labels expand the bbox)
• Confirms savefig(bbox_inches='tight') completes without error