fix(web): suppress Mermaid render error SVGs

[swear01]

Closes #785

Summary

• Suppress Mermaid's built-in error SVG rendering for invalid diagrams so failed Mermaid blocks do not inject the red "Syntax error in text" diagram into the page.
• Pre-validate Mermaid source with mermaid.parse(..., { suppressErrors: true }); invalid syntax falls back to the source block without calling render.
• Install a no-op Mermaid parse error handler to avoid global parse-error side effects.
• Add tests for valid rendering, invalid syntax fallback, and render-failure fallback without Mermaid error SVG injection.

Notes

• Related to feat(web): mermaid diagram lightbox on click #741 because it also touches Mermaid rendering, but this fixes a separate render-error/fallback path.

Test plan

• bun typecheck
• bun run test:cli — 93 files / 847 tests passed
• bun run test:hub — 263 tests passed
• bun run test:web — 89 files / 766 tests passed
• bun run test:shared — 68 tests passed
• bun run --cwd web test src/components/assistant-ui/mermaid-diagram.test.tsx — 3 tests passed
• Manual verification: reproduced the original environment showing Mermaid error SVG while Markdown still rendered, then confirmed the fix prevents that error SVG path.

[github-actions]

Findings
No high-confidence issues found in the added or modified lines.

Summary
Review mode: initial
No correctness, security, regression, data-loss, performance, or maintainability issues found at >=80% confidence. Residual risk: this was a static review only; Mermaid behavior was checked against published Mermaid 11.14.0 declarations/source metadata, but browser rendering was not exercised.

Testing
Not run (automation)

HAPI Bot