Harden chart node data, rendering, and lifecycle

[dovvnloading]

Summary

This PR repairs chart/graph-node data integrity, rendering behavior, graph lifecycle handling, and persistence.

What changed

• Added one canonical chart-data validator for all chart types.
• Rejects malformed containers, non-finite values, oversized payloads, invalid Sankey cycles, and mismatched series.
• Aggregates duplicate Sankey edges.
• Added responsive categorical tick selection and pie-slice grouping.
• Added render caching, font/theme-aware invalidation, resize bounds, and safer export cleanup.
• Added explicit chart provenance connections and recursive cleanup.
• Added stable chart IDs and parent/source references.
• Made chart restore fault-tolerant so one bad chart does not abort the entire chat.
• Prevented stale asynchronous chart results from landing in a different chat.
• Included chart structured data in search and chart-generation context.
• Added chart-focused regression coverage and updated search-highlight expectations.

Root cause

Chart data rules were duplicated between the agent, scene item, renderer, and persistence layers. Async insertion relied on the current node as a fallback, and charts were represented as loosely attached content without stable graph lifecycle metadata.

Validation

• python -m pytest -q
• Result: 408 passed, 1 existing asyncio deprecation warning
• Focused chart/scene tests also pass.