fix: support ParamWrapper types in multipart form value fields

[stareezy-1]

Summary

Fixes #958

Problem

ParamWrapper-based types work for query/header/path parameters but fail inside MultipartFormFiles[T] form value fields. A wrapper-backed value type (e.g. a big-int whose parsing is delegated to a text-unmarshalling wrapper via Receiver()) fails at request time:

422 Unprocessable Entity
"message": "unsupported param type: utils.BigInt"
"location": "form.step_id"

Root cause: the non-multipart parameter path unwraps ParamWrapper.Receiver() before calling parseInto, but the multipart form-value parser calls parseInto directly on the struct field. The wrapper value therefore never reaches the encoding.TextUnmarshaler check that the non-multipart path relies on.

Fix

Apply the same receiver unwrap in the multipart form-value path before parsing, mirroring the query/header/path parameter path:

receiver := f
if f.Addr().Type().Implements(paramWrapperType) {
    receiver = f.Addr().Interface().(ParamWrapper).Receiver()
}
pv, err := parseInto(ctx, receiver, value[0], value, *p)

Tests

TestMultipartFormFileWrapperParam added to huma_test.go, reproducing the issue shape: a value type implementing only Schema/String, with parsing delegated via Receiver() to a wrapper implementing UnmarshalText.

Behavior	Verifies
Without the fix	request fails with the reported unsupported param type 422
With the fix	form value parses; handler returns 200 with the expected payload

All existing tests continue to pass (go test -race ./...).