[DBA-94] Fix plot propagation to previous messages

[SimonKaran13]

Problem

When a user sends multiple queries that generate plots, new plots overwrite previously rendered messages. The root cause is that thread._visualization_result is a shared mutable singleton on the Thread object — every thread.plot() call overwrites it, so non-latest messages with has_visualization=True but no per-message visualization_data fall back to this shared state and render the wrong plot.

Solution

Restrict the shared thread fallback to the latest message only. Non-latest messages now render exclusively from their own persisted visualization_data. An allow_thread_fallback guard on render_visualization_section provides defense-in-depth, and a persistence fix ensures missing parquet files don't leave spec_df absent from the dict (which caused silent render failures).

Summary

• Restrict thread._visualization_result fallback to the latest message only
• Add allow_thread_fallback keyword-only parameter to render_visualization_section as defense-in-depth guard
• Fix persistence round-trip in load_chat when _has_spec_df is True but parquet file is missing
• Add diagnostic warning log when visualization_data is None after a successful thread.plot() call

Test plan

• make check passes (ruff, mypy, agent guidance)
• make test passes (pre-existing test_index failure only)
• 9 new regression tests in tests/test_plot_propagation.py
• Manual: send two queries that generate plots → first message's plot must not change after second completes
• Manual: click "Generate Plot" on an older message → only that message gets the new plot
• Manual: refresh browser → all plots render correctly for their respective messages

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