feat: add hover and color-scale support to DensityMapbox

[jqnatividad]

Summary

DensityMapbox previously exposed only a small subset of the plotly.js densitymapbox attributes — notably no hover configuration and no color-scale controls — so a density heatmap could not show per-point hover text or a custom/reversed colorscale + colorbar.

This PR adds those attributes, following the existing FieldSetter conventions and mirroring the field/type/serde choices already used by ScatterMapbox and ChoroplethMap. All existing fields are untouched, so the change is purely additive.

Closes #413.

What changed

Added Option<T> fields to DensityMapbox:

• Hover/text: text, hovertext, hoverinfo, hovertemplate, hovertemplatefallback, hoverlabel
• Color controls: colorscale, autocolorscale, reversescale, showscale, colorbar, coloraxis
• Data/misc: ids, meta, customdata, uirevision, below

The FieldSetter derive generates both scalar and _array setters (e.g. hover_text / hover_text_array), so per-point hover text and shared colorscales are now expressible. The two dead commented-out color_continuous_* placeholder lines were removed. The serialize_density_mapbox unit test is extended to cover the new keys.

Out of scope (intentionally not changed)

• radius stays Option<u8> (scalar). plotly.js allows number or array; widening to Dim<_> is a breaking change to the existing setter and left as a follow-up.
• The pre-existing zoom and line fields are not valid densitymapbox trace attributes (zoom belongs on layout.mapbox.zoom); removing them would be breaking, so they're left for a separate cleanup.

Testing

• cargo test -p plotly — serialize_density_mapbox passes with the new keys.
• cargo clippy --features all -- -D warnings -A deprecated — clean.
• cargo +nightly fmt --all -- --check — clean.

🤖 Generated with Claude Code

[jqnatividad]

Closing this. On reflection, adding hover/color-scale to densitymapbox isn't the right direction: plotly.js has deprecated the Mapbox-based traces in favor of the MapLibre map subplot, so the better home for these attributes is densitymap.

#416 adds a DensityMap trace with the same hover and color-scale surface (hovertemplate, colorscale/colorbar/showscale/reversescale/ autocolorscale/coloraxis, hoverlabel, etc.) on the non-deprecated trace, which covers the original need in #413. I'd rather not expand the API surface of a deprecated trace, so I'll let #416 stand as the successor and keep #413 open against that instead.