feat(plotting): Add group_cmaps to sc.pl.dotplot for custom per-group coloring

- 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 (dots overlapping in size legend, especially when dendrogram=True) by setting the legends width to 2.0 (compared to DEFAULT_LEGENDS_WIDTH = 1.5) inside the dotplot wrapper function.

- 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/3764.feature.md

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+- {func}`scanpy.pl.dotplot` now supports a `group_cmaps` parameter for custom per-group coloring.