refactor(lib): organizational changes to the package structure

[nhankyjangchan]

Work performed

• Extract Symbol.for('skip-override'), Symbol.for('fastify.display-name') and Symbol.for('plugin-meta') into lib/symbols.js, and re-export them from the entry point as fastifyPlugin.symbols.
• Move metadata normalization and validation into lib/prepareMetadata.js.
• Improve readability of lib/getPluginName.js and lib/toCamelCase.js.
• Add JSDoc annotations to all functions.
• Add FastifyPluginFunction type alias that covers both async and callback plugin signatures.
• Update error messages to include the actual type of the offending value (e.g. null, array, string) instead of a generic object message.
• Update tests to cover the new error messages and the extracted modules.
• Rename the second argument of plugin(fn, ...) from options to metadata, which better describes what it actually is: plugin metadata (name, fastify version, encapsulate), not generic options.

For what?

• Expose metadata symbols. kSkipOverride, kDisplayName and kPluginMeta are currently private to index.js, so consumers have to write Symbol.for('plugin-meta') and hope the string is correct — Symbol.for never throws on a typo. Exporting them as fastifyPlugin.symbols turns that string into an API.
• Separate validation from plugin wiring. Metadata normalization and validation live in index.js next to naming, symbols and camelCase conversion. Extracting them into lib/prepareMetadata.js makes the rules testable in isolation and shrinks plugin() to what it actually does.
• Make error messages actionable. 'The options object should be an object' does not say what the caller passed. The new message reports the actual type (null, array, string, …), which is what you need when a plugin fails to register.
• Name the argument for what it holds. The second argument of plugin(fn, ...) is not generic options — it is plugin metadata (name, fastify, encapsulate). metadata matches both the content and the existing plugin-meta symbol.
• Improve developer experience. JSDoc on all exported functions and a FastifyPluginFunction alias covering async and callback signatures make the public API self-describing.

Verified before pull request

• npm run lint:fix && npm run lint && npm run test passes
• All tests pass
• Code follows the project's formatting rules
• No breaking API changes; only the invalid-metadata error text changed

[Fdawgs]

Whilst i'm sure some of this is reasonable, squashing it all into an AI-generated commit and PR isn't.

[nhankyjangchan]

Whilst i'm sure some of this is reasonable, squashing it all into an AI-generated commit and PR isn't.

I only used AI to draft the PR text because I didn't want to embarrass myself with my English skills. I’m typing this message using a translator right now, so I hope it’s reasonably readable.

Does the code I submitted also look like "AI slop"? If you’re willing and able, could you point out my mistakes? I didn't use AI to write the code, but the fact that you thought I did suggests the quality is quite poor. I’d really appreciate any criticism—thank you!

(As I mentioned earlier, this text was written using a translator. I really hope this doesn't lead to any awkward misunderstandings.)