chore(server): follow-ups for Nuxt adapter's h3 and nuxt/server support #2883

Follow-ups from #2882, which lets the Nuxt adapter handle both the portable nuxt/server event (Nuxt 4.6+) and the h3 v1 event.

Deprecate the h3 path

• createEventHandler in packages/server/src/adapter/nuxt/handler.ts still has an h3 v1 branch (lazy import('h3')) for unwrapped handlers.
• Remove it when Nuxt 4 support is dropped (around Nuxt 5, Nitro v3 / h3 v2).
• At that point also remove the optional h3 peer dependency and the H3Event member of NuxtHandlerEvent.

Track the nuxt/server type

• NuxtHandlerEvent imports RequestEvent from nuxt/server, which only exists from Nuxt 4.6.
• On Nuxt 4.0 to 4.5 that import is unresolved and becomes any under skipLibCheck (a type error without it).
• Options: define a small local structural type, or raise the nuxt peer range to ^4.6.0.
• Revisit if Nuxt changes RequestEvent before Nuxt 5.