Update dependency fastify-multipart to v5.2.1

Editor assessment

Updating fastify-multipart to v5.2.1 is stated exactly, and chore(deps) matches the maintenance bump in type and scope. The subject fully identifies the change — package and target version — so the reader needs no diff. There is no body, appropriate for a plugin version update governed by upstream release notes, and no footers appear, none being owed. Hygiene is clean: imperative mood, no trailing period, compact subject, no pasted material, no junk tokens. The message does everything a single-line dependency update must do and nothing it should not.