This diagnostic is issued when the `initializedBy` option in `@clientInitialization` is not a valid `InitializedBy` value or is invalid for the client position.

## Impact

- **Area:** SDK client construction. Blocks invalid client initialization metadata that would produce an impossible root or sub-client creation pattern.
- **Not affected:** Service operations and request/response payload shapes are unchanged.

## ❌ Incorrect Usage

```typespec
@clientInitialization({
  initializedBy: InitializedBy.customizeCode | InitializedBy.parent, // `customizeCode` cannot be combined
})
namespace BlobClient {

}
```

## Diagnostic Message

TCGC reports:

```text
Invalid 'initializedBy' value. `InitializedBy.customizeCode` cannot be combined with other values.
```

## ✅ How to Fix

Use `InitializedBy.individually` for root clients, `InitializedBy.parent` or `InitializedBy.individually | InitializedBy.parent` for sub clients, or `InitializedBy.customizeCode` by itself.

```typespec
@clientInitialization({
  initializedBy: InitializedBy.customizeCode,
})
namespace BlobClient {

}
```
