This diagnostic is issued when the value passed to the `@access` decorator is not one of the allowed access levels.

## Impact

- **Area:** SDK visibility overrides. Blocks the invalid `@access` value from changing generated public/internal visibility.
- **Not affected:** Model structure, operation signatures, and wire payloads are unchanged.

## ❌ Incorrect Usage

```typespec
enum CustomAccess {
  private: "private",
}
@access(CustomAccess.private) // `"private"` is not a valid access value
model SecretModel {}
```

## Diagnostic Message

TCGC reports:

```text
Access value must be "public" or "internal".
```

## ✅ How to Fix

Pass `Access.public` or `Access.internal` to `@access` and remove any other enum member or literal value:

```typespec
@access(Access.internal)
model SecretModel {}
```
