fix: interpolate choice item content

[han4wluc]

Choice item content is never interpolated: "Hello ${variables.name}" shows the binding raw when read through choice.items[i].content in a choice layout.

Reproduce: a choice item whose content contains a variable binding (regression test).

Fix: string content fields are rendered with the choice layout's template data; other item fields are unchanged.

Tests: the regression checks the rendered text and the choice ID; the full suite (2,163 tests) and lint pass.