V2 chat shows TeX, Mermaid diagrams and chart payloads as raw text

[paullizer]

Problem

Three kinds of block that SimpleChat already produces are rendered as raw text in the V2 React chat (application/v2_ui).

1. Mermaid diagrams

This is the least obvious and arguably the most impactful. Azure Content Understanding already extracts document figures as Mermaid and writes them into page content as fenced blocks:

# application/single_app/functions_content_understanding.py:330
if figure_kind == 'mermaid' and isinstance(figure_content, str) and figure_content.strip():
    diagram_block = f"```mermaid\n{figure_content.strip()}\n```"

So a user who uploads a document containing a flowchart and asks about it gets diagram source pasted into the middle of the answer. Nothing renders it in either interface.

2. TeX

Models emit \( ... \), \[ ... \] and $$ ... $$. None of it renders, so any answer involving mathematics arrives as unreadable markup.

3. SimpleChart

The built-in chart action emits ```simplechart fences (functions_chart_operations.py:191). The classic interface renders these via static/js/chat/chat-inline-charts.js; V2 shows the raw JSON payload — often several kilobytes of it — in the middle of the reply.

Scope

V2 React UI only. The classic interface is unchanged.

Constraints

• No new remote browser code. Third-party browser libraries must be downloaded and committed to the repository so the bytes the browser executes are pinned and any change to them is a reviewable commit rather than a silent substitution. This rules out resolving them from npm at build time as well as loading them from a CDN at run time.
• The Content-Security-Policy in config.py (default-src 'self', script-src 'self', font-src 'self') must not be relaxed.
• Diagram and equation source is untrusted model output, so anything rendered from it needs an explicit sanitizer boundary — V2 currently uses dangerouslySetInnerHTML nowhere.
• A single $...$ must not be treated as inline maths, or ordinary prose about money ("costs $5 to $10 per user") renders as an equation.

Acceptance criteria

• $$...$$, \[...\] and \(...\) render as maths; a lone $ does not
• ```mermaid fences render as diagrams
• ```simplechart fences render as charts, with the underlying data reachable and an image download
• Fenced and inline code containing these delimiters is left untouched
• A reply that is still streaming does not repeatedly hand the renderer a half-written fence
• Diagrams and charts follow the light/dark theme
• Copying a message does not paste a raw chart payload
• All third-party browser code is committed under version control, with licences and pinned versions
• The CSP is unchanged

Resolution

Implemented in v0.261.019.

• Libraries vendored under application/v2_ui/public/vendor/<library>-<version>/: KaTeX 0.18.4, Mermaid 11.17.2, Chart.js 4.5.1, DOMPurify 3.4.14. All files verified against the registry's published SHA-256, licences preserved, and loaded on demand so a conversation with no maths, diagram or chart downloads none of them. No npm dependencies were added.
• remark-math was deliberately not used: CommonMark treats a backslash before ASCII punctuation as an escape, so \(x\) is already the literal text (x) by the time an mdast node exists and a post-parse plugin cannot see the delimiters. Maths instead reuses the placeholder mechanism the renderer already has for citations and masks.
• KaTeX runs with trust: false; Mermaid with securityLevel: 'strict', htmlLabels: false, no autostart and no icon packs. Both pass through DOMPurify before injection — the only two dangerouslySetInnerHTML sites in the SPA, enforced by test.
• The markdown renderer moved from MessageList.tsx into its own AssistantMarkdown.tsx.

Verified in a real browser across light and dark themes. Covered by functional_tests/test_v2_rich_rendering.py; the full V2 suite (18 files) passes.

Documentation: docs/explanation/features/REACT_V2_UI.md, docs/explanation/release_notes.md.