feat!: migrate action to native ESM and upgrade @actions/* to ESM majors

.github/copilot-instructions.md

@@ -2,7 +2,7 @@
 
 ## Project Overview
 
-This is a GitHub Action (`tediousjs/setup-sqlserver`) that installs SQL Server on Windows-based GitHub Action runners. It's built with TypeScript, compiled with `@vercel/ncc`, and runs on Node.js 24.
+This is a GitHub Action (`tediousjs/setup-sqlserver`) that installs SQL Server on Windows-based GitHub Action runners. It's built as a native ESM TypeScript module, bundled with [Rollup](https://rollupjs.org/), and runs on Node.js 24.
 
 ## Local Development
 
@@ -15,25 +15,27 @@ nvm use
 ## Build, Test & Lint
 
 ```sh
-npm run build        # ncc build + auto-generates README usage section from action.yml
-npm run test         # mocha + ts-node (runs all tests)
-npm run test:coverage # tests with nyc coverage
-npm run lint         # eslint across src/, misc/, test/
-npm run lint:fix     # eslint with auto-fix
+npm run build         # rollup bundles src/ to lib/main/index.js + auto-generates README usage section from action.yml
+npm run test          # runs node:test with experimental module mocks
+npm run test:coverage # tests with Node's native --experimental-test-coverage
+npm run lint          # eslint across src/, misc/, test/
+npm run lint:fix      # eslint with auto-fix
 ```
 
 Run a single test file:
 
 ```sh
-npx mocha -r ts-node/register './test/utils.ts'
+node --test --experimental-test-module-mocks --disable-warning=ExperimentalWarning './test/utils.ts'
 ```
 
 Run a single test by name:
 
 ```sh
-npx mocha -r ts-node/register './test/**/**.ts' --grep 'correctly returns for windows-2019'
+node --test --experimental-test-module-mocks --disable-warning=ExperimentalWarning --test-name-pattern='correctly returns for windows-2019' './test/**/*.ts'
 ```
 
+TypeScript is executed directly by Node 24 via native type-stripping — no transpiler (no `ts-node`, no `tsx`) is needed to run `.ts` files. This is why test runs are very fast and imports use `.ts` extensions.
+
 The build step (`npm run build`) also runs `npm run docs`, which regenerates the usage section in `README.md` from `action.yml` via `misc/generate-docs.ts`. If you change `action.yml` inputs, rebuild to keep the README in sync.
 
 ## Architecture
@@ -52,11 +54,18 @@ The build step (`npm run build`) also runs `npm run docs`, which regenerates the
 
 **Version registry:** `src/versions.ts` defines a `Map<string, VersionConfig>` with download URLs (exe/box or SSEI), optional update URLs, and OS compatibility constraints for each supported SQL Server version (2008–2025). SQL Server 2025+ uses the SSEI bootstrapper model (`sseiUrl`) instead of direct exe/box downloads.
 
-**Build output:** `@vercel/ncc` bundles everything into `lib/main/index.js`, which is what `action.yml` references. The `lib/` directory is committed to the repository. **Every commit must include up-to-date build output** — CI checks this by rebuilding and running `git diff-files --quiet`. Always run `npm run build` and commit the resulting changes to `lib/` and `README.md` before pushing.
+**Build output:** Rollup (config in `rollup.config.mjs`) bundles everything into `lib/main/index.js` as a minified ESM module, which is what `action.yml` references. The `lib/` directory is committed to the repository. **Every commit must include up-to-date build output** — CI checks this by rebuilding and running `git diff-files --quiet`. Always run `npm run build` and commit the resulting changes to `lib/` and `README.md` before pushing.
+
+**ESM module format:** The package is published as ESM (`"type": "module"` in `package.json`). Consequences for contributors:
+
+- Relative imports must use explicit file extensions. In this codebase we use `.ts` extensions (e.g., `import { foo } from './bar.ts'`), which Node 24 resolves directly via native type-stripping, and Rollup resolves via `@rollup/plugin-typescript`. TypeScript is configured with `allowImportingTsExtensions: true` and `rewriteRelativeImportExtensions: true` so the emitted types remain valid.
+- Prefer idiomatic ESM imports throughout: `import * as core from '@actions/core'` for namespace imports, named imports like `import { HttpClient } from '@actions/http-client'`, or default-function exports (e.g., `install-native-client.ts`). Do **not** re-export modules as mutable objects just to enable test stubbing — tests use `node:test`'s `mock.module()` (see below) which mocks modules at the module-resolution layer instead of mutating bindings.
 
 ## Conventions
 
-- **Testing:** Mocha + Chai + Sinon. Tests heavily stub `@actions/*` packages and module-level dependencies using `sinon.stub()`. Each test file mirrors its source file (e.g., `test/install.ts` tests `src/install.ts`).
+- **Testing:** Uses the built-in [`node:test`](https://nodejs.org/api/test.html) runner with `--experimental-test-module-mocks` and `node:assert/strict`. No Mocha, Chai, Sinon, or c8. Each test file mirrors its source file (e.g., `test/install.ts` tests `src/install.ts`).
+  - **Mocking pattern:** Create `mock.fn()` stubs at the top of the test file, call `mock.module(specifier, { namedExports, defaultExport })` for each dependency you need to replace, then `await import('../src/sut.ts')` to load the system-under-test against those mocks. Reset call history and implementations in `beforeEach` using `fn.mock.resetCalls()` and `fn.mock.mockImplementation(...)`. Because a given `mock.module()` call cannot be replaced mid-run, the tests share one mock per module across all cases and vary behaviour via `mockImplementation`/`mockImplementationOnce(impl, onCall)` — note that `mockImplementationOnce` without `onCall` defaults to call index `0`, so pass an explicit index when queueing sequential once-implementations.
+  - **Coverage caveat:** `--experimental-test-coverage` under-reports coverage for files that are `mock.module()`-replaced in sibling tests (the aggregator treats the mock shadow as an uncovered run of the real file). Every source file achieves 100% coverage when its own test file is run in isolation; treat the aggregated numbers as a lower bound while this feature is experimental.
 - **Commit messages:** Follow [Conventional Commits](https://www.conventionalcommits.org/) enforced by commitlint. Semantic-release uses these to generate automated releases and changelogs, so correct commit types are critical.
   - `fix` — Bug fixes or behavioural corrections that don't change public interfaces. Triggers a **patch** release.
   - `feat` — New backwards-compatible functionality (e.g., adding a new input, method, or option without removing anything). Triggers a **minor** release.

misc/generate-docs.ts

@@ -1,8 +1,12 @@
-import * as fs from 'fs/promises';
-import * as os from 'os';
-import * as path from 'path';
+import * as fs from 'node:fs/promises';
+import * as os from 'node:os';
+import * as path from 'node:path';
+import { fileURLToPath } from 'node:url';
 import * as yaml from 'js-yaml';
-import { version } from '../package.json';
+import pkg from '../package.json' with { type: 'json' };
+
+const { version } = pkg;
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
 /**
  * From @actions/checkout