fix(cookies): serialize maxAge of zero as Max-Age=0

[andrewstellman]

Problem

serializeCookie checks options.maxAge for truthiness, so maxAge: 0 emits no Max-Age attribute and the cookie becomes a session cookie instead of expiring:

response.plainCookie('k', 'v', { maxAge: 0 })
// Set-Cookie: k=...; Path=/; HttpOnly   (expected Max-Age=0)

Fix

Treat 0 as a value; undefined, null and '' behave as before.

One behaviour change to be aware of: an app whose config sets cookie: { maxAge: 0 } currently gets session cookies, and after this gets Max-Age=0 on every cookie that doesn't set its own maxAge.

Testing

• npm run quick:test (661 passed)
• npm run typecheck
• npx eslint and prettier --check on the changed files

Found by Quality Playbook, an AI code-review tool, with Claude; I reviewed the change.