Update dependency fastify-multipart to v4.0.1

Editor assessment

Updating fastify-multipart to v4.0.1 is stated precisely, with chore(deps) matching the maintenance bump in type and scope. The subject fully identifies the change — package and target version — so no diff is needed. There is no body, appropriate for a plugin version update, and no footers appear, none being owed. Hygiene is clean: imperative mood, no trailing period, compact subject, no pasted material, no junk tokens. A routine dependency update expressed in one complete line, with nothing vague, misleading, or padded anywhere.