Improve unsupported math fallback and scientific-unit guidance

[yaacovcorcos]

Problem

Model-generated scientific notation can contain a math-mode command inside a TeX text block, for example:

$$1.8\text{ \mu g/dL}$$

KaTeX rejects \mu in text mode. Scient then displays a code-styled fallback surrounded by manufactured $$ delimiters. The same fallback is used while loading and for render-boundary failures, so the visible result does not explain whether the source is unsupported, still loading, or failed for another reason.

Verified diagnosis

• Direct KaTeX rendering with Scient's options rejects 1.8\text{ \mu g/dL} with Undefined control sequence: \mu.
• The equivalent 1.8\,\mu\mathrm{g/dL} renders successfully.
• The stored message source contained the unsupported form; this was not caused by RTL layout or by the Markdown recognizer.
• renderScientTexToHtml returns null on the KaTeX error, and ScientMathLiteral supplies the generic fallback.

Relevant code:

• apps/web/src/scient/math/katexRuntime.ts
• apps/web/src/scient/math/ScientMath.tsx

Desired behavior

Unsupported TeX should remain readable and copyable, preserve its actual source and delimiter form, and be distinguishable from a temporary loading state. Scient's supported-format guidance should show valid scientific-unit notation.

Targeted scope

• Preserve the original inline/display delimiter form in the fallback rather than always manufacturing $$.
• Distinguish unsupported syntax from loading where that can be done without exposing internal errors in ordinary chat UI.
• Add supported examples for scientific units such as 1.8\,\mu\mathrm{g/dL}.
• Test inline and display math, streaming/loading, render errors, editing, and copying.

Non-goals

• Do not regex-rewrite arbitrary TeX.
• Do not silently move commands into or out of \text{...}.
• Do not treat this malformed source as an RTL rendering defect.

Evidence limits

The renderer failure is deterministic in automated reproduction. Native visual behavior still needs manual review before closure.