Setting default for non-string type with string schema panics

[costela]

This picks up from this discussion: #228 (comment)

Currently, using a custom type with different schema type and underlying go type panics.
In the example below, the schema type is string (via implementing encoding.TextUnmarshaler), but the go type is not a string.

package main

import (
	"context"
	"net/http"
	"time"

	"github.com/danielgtaylor/huma/v2"
	"github.com/danielgtaylor/huma/v2/adapters/humago"
)

type MyDuration struct {
	time.Duration
}

func (d *MyDuration) UnmarshalText(data []byte) error {
	v, err := time.ParseDuration(string(data))
	if err != nil {
		return err
	}
	d.Duration = v
	return nil
}

func main() {
	router := http.NewServeMux()
	api := humago.New(router, huma.DefaultConfig("My API", "1.0.0"))

	huma.Register(api, huma.Operation{
		OperationID: "demo",
		Method:      http.MethodGet,
		Path:        "/demo",
	}, func(ctx context.Context, i *struct {
		Duration MyDuration `json:"duration" query:"duration" default:"10s"` // default → 💥
	}) (*struct{}, error) {
		return nil, nil
	})

	http.ListenAndServe(":8888", router)
}

This leads to:

panic: unable to convert string to main.MyDuration for field 'Duration': schema is invalid

I suspect convertType must be extended to also recognize the same special-casing for types that can be converted to strings?