[Bug]: Body models for spread scenarios should be marked internal

[m-nash]

Describe the bug

Right now if you have a model like this

model Foo {
  x: int;
  @header y: string;
}

and you use this in an operation like this

op method(...Foo) : Foo;

We end up with a model FooRequest which only has the body param x which is fine but this type would never be used on the public api surface and therefore should default to access of internal. Today it comes through as public.

Reproduction

See above

Checklist

• [X] Follow our Code of Conduct
• [X] Check that this issue is about the Azure libraries for typespec. For bug in the typespec language or core libraries file it in the TypeSpec repo
• [X] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• [X] The provided reproduction is a minimal reproducible example of the bug.

[tadelesh]

it should have internal access, see the test here.