Introduce typed artifact system for explainer and explorer outputs

[Irozuku]

Summary

Introduces a typed artifact contract for all renderable component outputs. Explainers and explorers previously returned ad hoc shapes (lists of plotly JSON strings, {data, type, config} dicts), which hardwired the frontend to plotly and blocked explainers whose natural output is text, tables, or images. Every explainer plot() and explorer get_results() now returns a list of typed artifacts (plotly, table, text, image) with a single wire format {type, payload, title}, rendered by one shared frontend component. Legacy persisted results keep working through a normalization layer.

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Type of Change

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

• DashAI/back/core/artifacts.py: new pydantic artifact hierarchy (Artifact, PlotlyArtifact, TableArtifact with highlightable cells, TextArtifact, ImageArtifact with from_dashai_image bridge), from_dict via discriminated union, and normalize_artifacts upgrading legacy explainer strings and legacy explorer dicts.
• DashAI/back/explainability/global_explainer.py, local_explainer.py: plot() contract is now List[Artifact].
• DashAI/back/explainability/explainers/{kernel_shap,partial_dependence,permutation_feature_importance}.py: figures wrapped in PlotlyArtifact; KernelShap titles each instance ("Instance N").
• DashAI/back/job/explainer_job.py: pickles normalized artifact dicts (never live objects); local explanation file renamed .json -> .pickle (content was always pickle).
• DashAI/back/exploration/base_explorer.py + the 14 concrete explorers: get_results() returns artifacts (plotly explorers, describe/row tables built server side, wordcloud as native image).
• DashAI/back/api/api_v1/endpoints/{explainers,explorers,pipelines}.py: responses normalized, so old pickles and legacy plugin components still render.
• DashAI/front/src/components/shared/ArtifactRenderer.jsx: shared renderer for all four artifact types, grouping consecutive same-title artifacts per instance (plot + text summary shown together, one selector entry per instance).
• DashAI/front/src/utils/artifactVisualizerData.jsx: maps artifacts onto the existing explorer visualizers (plotly toolbar, material-react-table, image).
• DashAI/front/src/components/{explainers/ExplainersPlot,notebooks/explorer/useExplorerResults,explorations/explorers/DetailTabs/Results,pipelines/results/ResultsExploration}.jsx: consumers migrated to the artifact list response.
• DashAI/front/src/types/artifact.ts, api/{explainer,explorer}.ts: shared IArtifact type; response typings fixed.
• tests/back/core/test_artifacts.py: round-trip, validation, and normalization coverage (25 tests).

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Testing

• uv run pytest tests/back/ and cd DashAI/front && yarn build.
• Back compat: an explanation or exploration created before this branch must still render (normalization on the read path).
• Local explainer with many instances: selector shows one entry per instance; plot and text render stacked.

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Notes

• Plot files stay pickled (dicts only) so one read path serves old and new files; migrating to JSON files is deferred.
• The wire format is intentionally neutral: it is shared by explainers and explorers and is the input contract for the future story() responsibility.