feat!: pass the Fastify instance to all application hooks - #7219

Standardize how all application lifecycle hooks expose the associated Fastify instance so both function declarations and arrow functions have consistent access to the encapsulated scope (instance === this).

Motivation

Previously, application hooks exposed the Fastify instance inconsistently (#4967):

• onRoute, onReady, onListen, and preClose bound this to the Fastify instance without passing it as an argument, preventing arrow functions from accessing the encapsulated instance.
• onClose bound this and passed instance as the first argument.
• onRegister bound this to the parent instance where .register() was called, while passing the newly created child scope as the first argument (instance !== this).

Hook Signatures & this Binding Overview (v5 vs. v6)

• Application Hooks: Every application hook now binds this to the associated Fastify instance and passes that same instance (instance === this) as the first positional argument. Zero-arg and single-arg sync/async lifecycle handlers (() => {}, async () => {}, async (instance) => {}) continue to work seamlessly; callback-style handlers now take (instance, done).
• Request/Reply Hooks (lifecycleHooks): All 10 Request/Reply hooks (including onTimeout and onRequestAbort) already bind this to the route's encapsulated FastifyInstance and expose the same instance to arrow functions via request.server (and reply.server where reply is available), so their signatures remain unchanged (No changes).

Summary of Changes

• Runtime (lib/hooks.js, lib/plugin-override.js, lib/route.js, fastify.js):
  • Pass instance as the first argument in hookRunnerApplication (onReady, preClose), onListenHookRunner (onListen), onRoute, and onRegister.
  • Update callback arity detection (fn.length >= 2) and FST_ERR_HOOK_INVALID_ASYNC_HANDLER validation for the new parameter counts.
• TypeScript (types/hooks.d.ts, types/instance.d.ts, test/types/hooks.tst.ts):
  • Update callback and async hook handler types (onRouteHookHandler, onRegisterHookHandler, onReady*, onListen*, preClose*, onClose*) and preserve TypeProvider generic propagation on fastify.addHook(...).
• Tests & Documentation:
  • Add test/hooks-application-instance.test.js covering root, arrow functions, nested child/grandchild encapsulation, fastify-plugin, and 0/1-arg sync/async handlers, and assert instance === this across existing hook test suites (including this and request.server encapsulation checks on onTimeout and onRequestAbort).
  • Update docs/Reference/Hooks.md (covering both Application Hooks and this / request.server on Request/Reply Hooks), docs/Reference/TypeScript.md, docs/Guides/Plugins-Guide.md, and examples/hooks.js.

Closes #4967

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct