feat: Handle static file serve using fastify

[x42en]

This pull request introduces static file serving to the IOServer framework, enabling seamless support for single-page applications (SPAs) and static assets. The new functionality allows serving a directory of static files (such as a built frontend) and provides an optional SPA fallback mode, ensuring client-side routing works as expected. Comprehensive tests have been added to verify all static serving scenarios.

Key changes:

Static file serving and SPA fallback support

• Added a rootDir option to IOServerOptions and constructor, enabling serving of static files from a specified directory using @fastify/static. If the directory does not exist, static serving is safely disabled with a warning. (src/IOServer.ts, package.json) [1] [2] [3]
• Introduced a spaFallback option (default: true) that, when enabled with rootDir, serves index.html for any unmatched route—allowing SPA client-side routers to function correctly. If disabled, unmatched routes return the standard 404 JSON response. (src/IOServer.ts) [1] [2] [3]

Core implementation and API changes

• Registered @fastify/static plugin conditionally in the setupPlugins method and updated the not-found handler to support SPA fallback or default 404s as appropriate. (src/IOServer.ts) [1] [2]
• Updated the version to 2.1.0 to reflect the new feature. (package.json, src/IOServer.ts) [1] [2]

Testing

• Added a comprehensive test suite (IOServer.static.test.ts) covering all static file serving behaviors: constructor handling, static file and asset serving, SPA fallback logic, API route priority, and default 404 handling when static serving is disabled. (tests/unit/IOServer.static.test.ts)

These changes provide robust, production-ready static file serving for SPAs and static assets, with full test coverage and backward compatibility for existing API-only usage.