Fixes #28117

Previously, axis labels on 3D axes were excluded entirely from the tightbbox when for_layout_only=True. This caused savefig(bbox_inches='tight') to clip these labels in the saved output since bbox_inches='tight' uses get_tightbbox() internally. The inline Jupyter backend (%matplotlib inline) also triggers this path, which is the original report in #28117.

Root cause: In axis3d.py, the label was guarded by not for_layout_only:

if (self.label.get_visible() and not for_layout_only and
        self.label.get_text()):
    other.append(self.label.get_window_extent(renderer))

Fix: Harmonize with 2D axis behavior (axis.py lines 1368–1380) always include the label bbox, but collapse it in the irrelevant direction when for_layout_only=True.

Minimum reproducible example:

import matplotlib.pyplot as plt

fig = plt.figure(figsize=(4, 3))
ax = fig.add_subplot(1, 1, 1, projection='3d')
ax.plot([0, 1], [0, 1], [0, 1])
ax.set_zlabel("Z axis label")
ax.set_xlabel("X axis label")
fig.savefig("out.png", bbox_inches='tight')

Before fix: zlabel and xlabel are clipped in the saved image. After fix: all labels are fully visible.

AI Disclosure

None.

PR checklist

• closes [Bug]: The zlabel on 3D axes will be cut when using '%matplotlib inline' in Jupyter #28117
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines