feat(plotting): Add group_cmaps to sc.pl.dotplot

- Adds the group_cmaps parameter to sc.pl.dotplot, allowing users to specify a unique colormap for each group.

- Implements a corresponding stacked colorbar legend (_plot_stacked_colorbars) to display the multiple colormaps, with group labels for clarity.

- Adds robust error handling, raising a ValueError if a plotted group is not defined in the group_cmaps dictionary.

- Fixes a pre-existing layout bug by increasing DEFAULT_LEGENDS_WIDTH from 1.5 to 1.7, which prevents circles in the size legend from overlapping.

- Refactors data preparation logic from DotPlot.__init__ into a new _prepare_dot_data helper method. This was done to improve code quality and resolve pre-commit linter errors related to complexity.

- Includes comprehensive tests for the new functionality, swap-axis compatibility, and error conditions.

Author: rwbaber

docs/release-notes/dotplot_group_cmaps.feature.md

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+- Added `group_cmaps` parameter to `sc.pl.dotplot` for custom per-group coloring.