fix: preserve cache error class names in save-cache bundle (#1093)

## Summary

- Preserve class names in the save-cache bundle so `@actions/cache`
error-name comparisons distinguish validation errors, benign
cache-reservation races, and finalization errors correctly.
- Add regression coverage that exercises the bundled `ValidationError`,
`ReserveCacheError`, and `FinalizeCacheError` constructors.

Fixes #1028

## Verification

- `npm run all` passed on Ubuntu 24.04 with Node 24.19.0: build, Biome,
packaging, and all 159 tests across 15 suites.
- `git diff --check` passed.

The save-cache bundle increases by about 77 KB raw (14 KB gzip) to
preserve the required names.

__tests__/cache/save-cache-bundle.test.ts

@@ -0,0 +1,33 @@
+import { readFileSync } from "node:fs";
+import { runInNewContext } from "node:vm";
+import { expect, it } from "@jest/globals";
+
+const saveCacheBundle = readFileSync(
+  new URL("../../dist/save-cache/index.cjs", import.meta.url),
+  "utf8",
+);
+
+function bundledErrorClass(name: string): new (message: string) => Error {
+  const start = saveCacheBundle.indexOf(`var ${name} = class`);
+  expect(start).toBeGreaterThanOrEqual(0);
+
+  const end = saveCacheBundle.indexOf("\n};", start);
+  expect(end).toBeGreaterThan(start);
+
+  const declaration = saveCacheBundle.slice(start, end + 3);
+  return runInNewContext(`${declaration}; ${name}`, {
+    __name: (target: object, value: string) =>
+      Object.defineProperty(target, "name", { configurable: true, value }),
+  }) as new (
+    message: string,
+  ) => Error;
+}
+
+it.each(["ValidationError", "ReserveCacheError", "FinalizeCacheError"])(
+  "preserves the %s name in the save-cache bundle",
+  (name) => {
+    const ErrorClass = bundledErrorClass(name);
+
+    expect(new ErrorClass("test").name).toBe(ErrorClass.name);
+  },
+);

scripts/build-dist.mjs

@@ -9,6 +9,8 @@ const builds = [
   },
   {
     entryPoints: ["src/save-cache.ts"],
+    // @actions/cache compares error.name to the error constructor's name.
+    keepNames: true,
     outfile: "dist/save-cache/index.cjs",
     staleOutfiles: ["dist/save-cache/index.mjs"],
   },