[Backport 5.x] fix: copy a set-cookie array passed to reply.header instead of storing it - #7087

…g it (#7068)

`reply.header('set-cookie', value)` stores `value` in the reply's header
map. When `value` is an array, the array object itself is stored, and
every subsequent `set-cookie` call on the reply pushes into it
(`lib/reply.js`, the `Array.prototype.push.apply` / `.push` branch). The
string case already copies into a fresh array before pushing, so only
the array case leaks the caller's object.

If the array is shared, for example a module-level default, it grows by
one entry on every request, and responses carry more and more duplicate
cookies:

```js
const defaults = ['a=1']
fastify.get('/', (req, reply) => reply.header('set-cookie', defaults).header('set-cookie', 'b=2').send({}))
// 1st response: a=1, b=2   2nd: a=1, b=2, b=2   3rd: a=1, b=2, b=2, b=2 ...
```

`reply.headers({ 'set-cookie': defaults })` goes through the same path.

This PR stores a copy of the array, so later pushes stay local to the
reply.

## Checklist

- [x] run `npm run test` and `npm run benchmark` —
`test/internals/reply.test.js` 86/86 with the change; the new test
`reply.header does not mutate a set-cookie array passed by the caller`
fails on `main` (the second response carries `b=2` twice and the
caller's array has grown) and passes with this change. `eslint` is
clean.
- [x] tests and/or benchmarks are included
- [x] documentation is changed or added — n/a, behaviour now matches the
documented one
- [x] commit message follows [commit
guidelines](https://github.com/fastify/fastify/blob/main/CONTRIBUTING.md#commit-guidelines)

Signed-off-by: Fadi Romdhan <[email protected]>
Co-authored-by: Fadi Romdhan <[email protected]>
(cherry picked from commit 5c081d4)