fillTemplates: substituteFields skips in-string substitution for numeric/boolean field values

[JamesPHoughton]

Bug

`substituteFields` in server/src/preFlight/fillTemplates.js only does in-string ${key} replacement when the field value is a string. Numeric and boolean values silently fall through the embedded-string path, so a YAML-native int like roundN: 1 leaves ${roundN} literal inside longer strings such as round_${roundN}_choice. The whole-value "${key}" regex (which JSON-stringifies) already works for all types — only the embedded-string path is missing.

The in-line comment already describes the intended behaviour as "string or number" — only the implementation forgot the number case:

// if the value is just a string or number, we can also replace instances of ${key} within other strings
if (typeof value === "string") {   // ← should also accept number / boolean
  const stringReplacementRegex = new RegExp(`\\$\\{${key}\\}`, "g");
  stringifiedTemplate = stringifiedTemplate.replace(
    stringReplacementRegex,
    value,
  );
}

Repro

substituteFields({
  templateContent: { name: "round_${roundN}_choice" },
  fields: { roundN: 1 },
});
// → { name: "round_${roundN}_choice" }   ← bug, placeholder survives

With roundN: "1" (string), substitution works. With roundN: 1 (number), it doesn't.

This was originally surfaced in deliberation-lab/stagebook (which has its own copy of fillTemplates) when a Prisoner's Dilemma example used a ${roundN} broadcast over numeric round IDs and the viewer prompted for a roundN field at study start. We fixed it there in deliberation-lab/stagebook#276 — same fix should apply here.

Suggested fix

Extend the type check to cover all stringifiable scalars and use String(value) to handle non-string values cleanly:

if (
  typeof value === "string" ||
  typeof value === "number" ||
  typeof value === "boolean"
) {
  const stringReplacementRegex = new RegExp(`\\$\\{${key}\\}`, "g");
  stringifiedTemplate = stringifiedTemplate.replace(
    stringReplacementRegex,
    String(value),
  );
}

The whole-value "${key}" replacement above this block already handles object/array values via JSON.stringify, so this branch only needs to cover scalars that can meaningfully be embedded in a string. Booleans get the same treatment for symmetry — ${flag} → "true" / "false".

Suggested regression tests (mirroring what we added in stagebook):

• Numeric field embedded in a string: { count: "${num}" } + { num: 42 } → { count: 42 } (already works), but { name: "x_${num}_y" } + { num: 42 } → { name: "x_42_y" } (the regression).
• Numeric broadcast field substituted into stage names — exact repro shape.
• Boolean field embedded in another string for symmetry.