This diagnostic is issued when the enum passed to `@clientApiVersions` omits one or more versions defined by the versioned service.

## Impact

- **Area:** Client API-version enum generation. Blocks a client API version list that would omit service versions required for compatibility.
- **Not affected:** The service's versioned namespace and declared service versions are unchanged.

## ❌ Incorrect Usage

```typespec
@service
@versioned(Versions)
namespace My.Service {
  enum Versions {
    v1,
    v2,
    v3,
  }
}

enum ClientApiVersions { // missing service version `v3`
  v1,
  v2,
}
@@clientApiVersions(My.Service, ClientApiVersions);
```

## Diagnostic Message

TCGC reports:

```text
The @clientApiVersions decorator is missing one or more versions defined in My.Service. Client API must support all service versions to ensure compatibility. Missing versions: v3. Please update the client API to support all required service versions.
```

## ✅ How to Fix

Add the missing service versions to the client API versions enum.

```typespec
@service
@versioned(Versions)
namespace My.Service {
  enum Versions {
    v1,
    v2,
    v3,
  }
}

enum ClientApiVersions {
  v1,
  v2,
  v3,
}
@@clientApiVersions(My.Service, ClientApiVersions);
```
