fix: propagate return value from fastify.errorHandler

[abdulmunimjemal]

Fixes #581.

Problem

The wrapper errorHandler registered on the static routes calls
fastify.errorHandler(error, request, reply) but discards its return
value (index.js:79).

That breaks custom error handlers that rely on Fastify's normal contract
of returning a value (or a Promise) instead of calling reply.send
themselves. In particular, an async error handler that returns its
payload never resolves the reply, because the dropped promise is never
awaited by Fastify.

Vanilla Fastify routes propagate that return value, so this was a
behavioral discrepancy specific to @fastify/static.

Fix

Return the value from fastify.errorHandler(...) so the application's
error handler can short-circuit / transform the reply exactly like it
would on a non-static route. One-line change.

Tests

Added a focused regression test in test/static.test.js that:

• Registers an async setErrorHandler returning its payload (without
  calling reply.send).
• Triggers an error from a preHandler so the static route's wrapper
  errorHandler runs.
• Asserts the status code, content-type, and body returned by the user's
  error handler reach the client.

Without the fix this test times out (the dropped promise never resolves
the reply). With the fix it passes.

Checks

• npm test — 320/320 passing, 100% line coverage retained.
• npm run lint — clean.

[mcollina]

lgtm