Support marginal_x/marginal_y="heatmap" in density_heatmap/density_contour

[lucasjamar]

Summary

• Adds 'heatmap' as a supported marginal_x/marginal_y value for density_heatmap/density_contour, drawing a single-row (marginal_x) or single-column (marginal_y) heatmap strip in the margin, colored by the same z/histfunc aggregate as the main plot (or raw count when z isn't given).
• Implemented as a go.Histogram2d trace reusing the real perpendicular-axis column, collapsed to a single explicit bin (via computed xbins/ybins edges — nbins*=1 alone isn't reliable since plotly.js's "nice number" bin-sizing can still round to 2 bins). Binning happens client-side, same as the main trace.
• Shares the main plot's coloraxis/colorbar (coloraxis1) rather than drawing its own. For density_contour, whose main trace is lines-only (contours.coloring="none") and has no existing coloraxis, coloraxis1 is now built (from a default colorscale — density_contour has no color_continuous_scale argument) whenever a heatmap marginal is requested, without changing the main contour trace's own appearance.
• Restricted to density_heatmap/density_contour — raises a clear ValueError on scatter/histogram/ecdf.
• Fixes two related bugs surfaced while testing: (1) the existing marker-color post-processing for marginals would crash on go.Histogram2d (no marker property), (2) density_contour's discrete color-grouping (line.color) update would crash when a go.Histogram2d marginal trace is present in the same group.

Fixes #5706

Test plan

• Added test_marginal_heatmap_uses_z_and_histfunc, test_marginal_heatmap_without_z, test_marginal_heatmap_unsupported_chart_type_raises, test_marginal_heatmap_with_discrete_color in tests/test_optional/test_px/test_marginals.py
• pytest tests/test_optional/test_px/test_marginals.py — 149 passed
• pytest tests/test_optional/test_px/ — 1139 passed, 3 skipped, 4 xfailed (47 pre-existing failures in test_trendline.py unrelated to this change — local statsmodels/scipy version conflict)
• ruff check clean on changed files (pinned ruff==0.11.12)
• Manually verified in-browser: single clean row/column strips (no double-row rendering), correct shared colorbar label/scale for both histfunc="sum" and histfunc="avg", and the density_contour + discrete color + heatmap marginal crash scenario