Primitive-based types are not reused in the generated OAS.

Hey there!

I've got the following types

```go

var _ huma.SchemaTransformer = CustomHeader("")

type CustomHeader string

func (h CustomHeader) TransformSchema(r huma.Registry, s *huma.Schema) *huma.Schema {
s.Type = "string"
s.Description = "Custom header Description. The CustomHeader schema should be reused across all endpoints that use it."
return s
}

type CreateUser struct {
Body struct {
Name string `json:"name" doc:"The name."`
}

CustomHeader CustomHeader `header:"X-Custom-Header"`
}

type UpdateUser struct {
Body struct {
Name string `json:"name" doc:"The name."`
}

CustomHeader CustomHeader `header:"X-Custom-Header"`
}

type Output struct {
Body struct {
Message string `json:"message" doc:"The message."`
}
}
```

With the following routes
```go
huma.Post(api, "/user", func(ctx context.Context, req *CreateUser) (*Output, error) {
resp := &Output{}
resp.Body.Message = "It works!"
return resp, nil
})

huma.Patch(api, "/user/{id}", func(ctx context.Context, req *UpdateUser) (*Output, error) {
resp := &Output{}
resp.Body.Message = "It works!"
return resp, nil
}}
```

I expect that `CustomHeader` would be re-used via `$ref` in the generated OAS.

Instead the `CustomHeader` schema is embedded into all the endpoints

```yaml
paths:
/user:
post:
operationId: post-user
parameters:
- description: Custom header Description. The CustomHeader schema should be reused across all endpoints that use it.
in: header
name: X-Custom-Header
schema:
description: Custom header Description. The CustomHeader schema should be reused across all endpoints that use it.
type: string
requestBody:
content:
application/json:
schema:
$ref: "#/components/schemas/CreateUserBody"
required: true
responses:
"200":
content:
application/json:
schema:
$ref: "#/components/schemas/OutputBody"
description: OK
default:
content:
application/problem+json:
schema:
$ref: "#/components/schemas/ErrorModel"
description: Error
summary: Post user
/user/{id}:
patch:
operationId: patch-user-by-id
parameters:
- description: Custom header Description. The CustomHeader schema should be reused across all endpoints that use it.
in: header
name: X-Custom-Header
schema:
description: Custom header Description. The CustomHeader schema should be reused across all endpoints that use it.
type: string
requestBody:
content:
application/json:
schema:
$ref: "#/components/schemas/UpdateUserBody"
required: true
responses:
"200":
content:
application/json:
schema:
$ref: "#/components/schemas/OutputBody"
description: OK
default:
content:
application/problem+json:
schema:
$ref: "#/components/schemas/ErrorModel"
description: Error
summary: Patch user by ID
```

Can an operation be registered to force Huma to reuse shared schemas?

Thank you.