Setting default for non-string type with string schema panics

This picks up from this discussion: https://github.com/danielgtaylor/huma/issues/228#issuecomment-1946513763

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.
```go
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?