build: update npm dependencies (#689)

## Summary

Update all direct npm dependencies and devDependencies to the current
npm latest versions and pin them exactly instead of using caret or tilde
ranges.

## Changes

- Pin all direct runtime and development package versions in
`package.json` and refresh `package-lock.json`.
- Migrate ESLint from `.eslintrc.json` to flat config for ESLint 10.
- Update Jest and TypeScript configuration and test mocks to support the
latest ESM-only packages.
- Add response status handling and typed JSON access for the latest
`node-fetch` types.

## Checklist

- [x] I have read the latest README and followed the instructions.
- [x] I have added or updated tests for behavior changes.
- [x] I have updated README.md and action.yml when inputs or runtime
behavior changed.
- [x] I have run the relevant verification commands.

## Verification

- [x] `npm --userconfig=/private/tmp/empty-npmrc outdated --json
--include=dev` returned `{}`
- [x] `npm run all`
- [x] `npm run build`


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Added comprehensive test mocks for Actions APIs and fetch, improved
integration tests, and updated assertions for more precise error
validation.
  * Configured Jest to map external modules to local test mocks.

* **Chores**
* Migrated ESLint to the flat config format and replaced prior ESLint
config.
* Updated project dependencies and modernized TypeScript compiler
settings.

* **Refactor**
* Adjusted exported tool constants and made error handling more explicit
in version-fetching logic.

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/peaceiris/actions-hugo/pull/689)
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Codex <noreply@openai.com>

__tests__/mocks/actions-exec.ts

@@ -0,0 +1,36 @@
+import {spawn} from 'child_process';
+
+interface ExecOptions {
+  listeners?: {
+    stdout?: (data: Buffer) => void;
+    stderr?: (data: Buffer) => void;
+  };
+}
+
+export async function exec(
+  commandLine: string,
+  args: string[] = [],
+  options: ExecOptions = {}
+): Promise<number> {
+  if (commandLine === 'hugo') {
+    const version = process.env.TEST_HUGO_VERSION || '';
+    const extended = process.env.TEST_HUGO_EXTENDED === 'true' ? ' extended' : '';
+    options.listeners?.stdout?.(Buffer.from(`hugo v${version}${extended}\n`));
+    return 0;
+  }
+
+  return new Promise((resolve, reject) => {
+    const child = spawn(commandLine, args);
+
+    child.stdout?.on('data', (data: Buffer) => {
+      options.listeners?.stdout?.(data);
+    });
+    child.stderr?.on('data', (data: Buffer) => {
+      options.listeners?.stderr?.(data);
+    });
+    child.on('error', reject);
+    child.on('close', code => {
+      resolve(code ?? 0);
+    });
+  });
+}