Clone ncols, shadow, fancybox, framealpha, title, loc, mode, borderaxespad and bbox_to_anchor from the existing legend before regenerating, so custom legend state is not discarded.

Closes #17775

PR summary

When the "(Re-)Generate automatic legend" checkbox is used in the Qt
figure options dialog, the existing code only preserved ncols and
draggable. All other custom legend settings (shadow, fancybox,
framealpha, title, loc, mode, borderaxespad, bbox_to_anchor) were
silently discarded.

This fix reads all relevant properties from the existing legend before
calling axes.legend() again, so the regenerated legend matches the
original state.

Minimum reproducible example:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot(range(5), label='a')
ax.plot(range(3)[::-1], label='b')
ax.legend(bbox_to_anchor=(0,1.02,1,0.2), loc="lower left", 
          mode="expand", borderaxespad=0, ncols=3)
plt.show()
# Open figure options, tick "(Re)-generate legend", click OK
# Legend settings are lost

AI Disclosure

Used Claude (Anthropic) to assist with identifying the correct
attributes to read from the Legend object and to verify the fix
via a test script.

PR checklist

• "closes clone more legend settings when regenerating in Qt figure options dialog #17775" is in the body of the PR description
• 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
• [N/A] Documentation complies with general and docstring guidelines