[Plugin API] Show rendered template-function output in the chip (like Insomnia)

Today, custom template-function plugins always render as name(…) in the chip. The previewArgs field can show input arg values, but there’s no way to display the onRender output. Insomnia shows the resolved value (e.g. DateAdd (2026-07-30T09:00)), which is much easier to scan in a request with several dynamic fields.

Proposal: either a previewLabel?: (ctx, args) => string callback on TemplateFunction, or a sentinel like previewArgs: ‘*output’ that displays the onRender return value.