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.

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"],
   },