Replaces four scattered dispatch locations (schema_validator, dataset_validator,
chart_utils, runtime validator) with a central ChartTypePlugin registry. Each of
the 7 supported chart types (xy, table, pie, pivot_table, mixed_timeseries,
handlebars, big_number) now owns its pre-validation, column extraction, form_data
mapping, post-map validation, column normalization, and runtime warnings in a
single plugin class.
Key changes:
- Add ChartTypePlugin protocol and BaseChartPlugin base class (plugin.py)
- Add ChartTypeRegistry with register/get/all_types helpers (registry.py)
- Add 7 chart type plugins under chart/plugins/ with full coverage
- Fix 5-type column validation gap: pie, pivot_table, mixed_timeseries, handlebars,
and big_number now participate in dataset column validation (previously silently skipped)
- Move BigNumber trendline temporal check to BigNumberChartPlugin.post_map_validate()
- Add get_runtime_warnings() to plugin protocol; XYChartPlugin implements
format/cardinality checks, removing isinstance(config, XYChartConfig) from RuntimeValidator
- Fix stale generate_chart.py docstring listing only 'xy' and 'table' chart types
- Add missing pie, pivot_table, mixed_timeseries handlers to _enhance_validation_error;
refactor into a data-driven lookup table to stay within complexity limits
- Fix empty details fallback in Pydantic error handler
