…autocolorscale=True

When a user explicitly provides color_continuous_scale, it should always be
respected, even if the template has coloraxis_autocolorscale=True. Previously,
the template's autocolorscale setting would override the user's explicit
colorscale.

The fix tracks whether color_continuous_scale was explicitly provided by the
user (before apply_default_cascade fills it from template/defaults), and only
sets autocolorscale=False when the user explicitly provided a colorscale. This
preserves automatic diverging palette selection when colorscale comes from
template/defaults.

Changes:
- plotly/express/_core.py: Track user_provided_colorscale and conditionally
  set autocolorscale=False only when user explicitly provides colorscale
- plotly/express/_imshow.py: Same fix for imshow() function

Add tests to verify that user-provided color_continuous_scale overrides
template autocolorscale=True, and that template autocolorscale is respected
when user doesn't provide a colorscale.

Changes:
- tests/test_optional/test_px/test_colors.py: Add test_color_continuous_scale_autocolorscale()
- tests/test_optional/test_px/test_imshow.py: Add test_imshow_color_continuous_scale_autocolorscale()