refactor(tests): centralize test harness boilerplate into bundle.mjs helper

tests/helpers/bundle.mjs

@@ -0,0 +1,55 @@
+// Shared test-harness bundler.
+//
+// Every run-*-test.mjs used to hand-roll the same esbuild boilerplate:
+//   write temp entry (optional) -> esbuild.build(bundle/node/esm/silent)
+//   -> dynamic import of the artifact -> rmSync cleanup.
+// This module is that sequence, parameterized. Harnesses call it once (or
+// more, like the chapters harness's three bundles) and get back the loaded
+// module plus a scoped cleanup function.
+import esbuild from "esbuild";
+import fs from "fs";
+import os from "os";
+import path from "path";
+import url from "url";
+
+export async function bundleForTest({
+  // Unique-ish name; also names the temp artifacts (and the generated entry
+  // file when entrySource is used).
+  name,
+  // Module paths to bundle directly (relative to the project root or absolute).
+  entryPoints,
+  // Alternative to entryPoints: inline TS re-export text, written to a temp
+  // entry file next to the project root (deleted right after the build).
+  entrySource,
+  format = "esm",
+  external = [],
+} = {}) {
+  const ext = format === "cjs" ? "cjs" : "mjs";
+  const out = path.join(os.tmpdir(), `${name}-${process.pid}.${ext}`);
+  let tempEntry = null;
+  let eps = entryPoints ?? [];
+  if (entrySource !== undefined) {
+    tempEntry = path.resolve(`.${name}-entry.ts`);
+    fs.writeFileSync(tempEntry, entrySource);
+    eps = [tempEntry];
+  }
+  await esbuild.build({
+    entryPoints: eps.map((p) => path.resolve(p)),
+    bundle: true,
+    platform: "node",
+    format,
+    outfile: out,
+    external,
+    write: true,
+    logLevel: "silent",
+  });
+  if (tempEntry) fs.rmSync(tempEntry, { force: true });
+  const mod = await import(url.pathToFileURL(out).href);
+  return {
+    mod,
+    // Removes only this call's bundled artifact.
+    cleanupBundle() {
+      fs.rmSync(out, { force: true });
+    },
+  };
+}