fix: run preClose hook exactly once per declaring instance - #6940

What

A single preClose hook added to a Fastify instance is executed once for every instance in the encapsulation subtree (the declaring instance plus each descendant plugin) instead of exactly once.

Reproduction

const fastify = Fastify()
let count = 0

fastify.register(async (child) => {
  child.get('/x', async () => 'ok')
})
fastify.addHook('preClose', async () => { count++ })

await fastify.ready()
await fastify.close()

console.log(count) // 2, expected 1

With two levels of nested plugins the same hook runs 3 times. An equivalent onReady hook in the identical setup correctly runs once.

Root cause

In addHook, the hooks onReady, onListen, and onRoute are special-cased to be stored only on the declaring instance via this[kHooks].add. preClose was omitted from that list, so it falls into the else branch and goes through _addHook, which copies the hook into every child via this[kChildren].forEach. At close time, hookRunnerApplication('preClose') already walks the entire child tree, so each propagated copy fires again.

That preClose is not meant to be inherited is already established elsewhere in the codebase: buildHooks in lib/hooks.js resets onReady, onListen, and preClose to empty arrays for children. preClose was simply left off the special-case list in addHook.

Fix

Add preClose to the same special-case branch as onReady/onListen/onRoute so it is stored only on the declaring instance and not propagated into children. Since hookRunnerApplication already recurses the child tree and buildHooks already excludes preClose from inheritance, each declared hook now fires exactly once.

Tests

Added two regression tests in test/close.test.js (a single child plugin and nested child plugins) asserting the hook runs exactly once. Both fail on main (count 2 and 3) and pass with this change. The full test/close.test.js suite (26 tests) stays green.

Checklist

• run npm run test and the linter
• tests are included
• commit is signed off (DCO)