Function makeMultipartBodyParser

Opt-in multipart/form-data body parser step.

@middy/http-multipart-body-parser is an optional peer dependency, so it is imported lazily on the first request that has a body: consumers who never enable this step need not install it. The parsed body is passed through unchanged (no reshaping); limits are left to the parser's own defaults and any busboy.limits the caller supplies.

The .catch is load-bearing: bundlers such as esbuild (used by the Serverless Framework) fail the build on an unresolvable bare import(), but leave it as a runtime import when the promise is handled. It also turns a missing package into an actionable error instead of a bare ERR_MODULE_NOT_FOUND.