fix(test): make the ToS heading guard actually require the parentheses (#13228)

CodeQL js/useless-regexp-character-escape (#994-#997) on one line, and it is a
real defect rather than the usual query noise.

The assertion built its pattern in a TEMPLATE literal:

    new RegExp(`\(\s*${String(tos?.actual)}\s*\)`)

JavaScript resolves the escapes before RegExp ever sees the string: `\(` becomes
"(" and `\s` becomes the LETTER "s". The compiled pattern was `(s*16s*)` — a
capture group around optional "s" characters — so it matched any heading merely
CONTAINING the number. The literal parentheses this guard exists to require were
never checked, and it passed on exactly the headings it was written to reject:

    /(s*16s*)/.test("### Caution — clauses worth checking 16")   // true

Doubled the backslashes so they survive the template literal, and routed the
interpolated value through an `escapeRegExp` helper — the count is a number
today, but interpolating an unescaped value into a regex source is the same
class of bug one refactor away.

Added a second test that pins the behaviour rather than the spelling: the
pattern must REJECT a heading carrying the count without parentheses, and accept
it with them (including inner whitespace). Before this fix that test fails.

4/4 green against the real docs/reference/FREE_TIERS.md heading.

tests/unit/check-docs-counts-tos-heading.test.ts

@@ -13,6 +13,11 @@ type Check = {
   validate?: (content: string, claim?: string) => { ok: boolean; detail: string };
 };
 
+/** Escape a value that is interpolated into a RegExp source. */
+function escapeRegExp(value: string): string {
+  return value.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
 describe("ToS caution heading count", () => {
   it("the Caution heading carries the count the gate checks", () => {
     const txt = readFileSync(join(process.cwd(), "docs/reference/FREE_TIERS.md"), "utf8");
@@ -21,7 +26,24 @@ describe("ToS caution heading count", () => {
     const tos = (buildChecks() as Check[]).find((c) =>
       String(c.docKey ?? "").includes("ToS caution")
     );
-    assert.match(heading, new RegExp(`\(\s*${String(tos?.actual)}\s*\)`));
+    // The backslashes must survive the TEMPLATE LITERAL to reach the regex.
+    // Written as `\(\s*…` they did not: JS resolves `\(` to "(" and `\s` to the
+    // LETTER "s" before RegExp ever sees them, so the pattern compiled to
+    // `(s*16s*)` — a capture group around optional "s" characters. That matched
+    // any heading merely containing the number, with no literal parentheses
+    // required at all, so this guard passed on headings it was written to reject
+    // (CodeQL js/useless-regexp-character-escape #994-#997).
+    const count = escapeRegExp(String(tos?.actual));
+    assert.match(heading, new RegExp(`\\(\\s*${count}\\s*\\)`));
+  });
+
+  it("the heading guard actually requires the parentheses", () => {
+    // Pins the defect above: the pattern this test builds must REJECT a heading
+    // that carries the count without parentheses. Before the fix it accepted it.
+    const pattern = new RegExp(`\\(\\s*${escapeRegExp("16")}\\s*\\)`);
+    assert.equal(pattern.test("### Caution — clauses worth checking 16"), false);
+    assert.equal(pattern.test("### Caution — clauses worth checking (16)"), true);
+    assert.equal(pattern.test("### Caution — clauses worth checking ( 16 )"), true);
   });
   it("buildChecks exposes a soft ToS entry on FREE_TIERS.md with requireClaim", () => {
     const checks = buildChecks() as Check[];