Previously, the Gemini per-event mapper shared by Zed-hosted models and
BYOK recorded `finishReason`, but did not emit
`LanguageModelCompletionEvent::Stop` for ordinary text completions such
as:

```json
{
  "candidates": [
    {
      "index": 0,
      "content": {
        "role": "model",
        "parts": [
          {
            "text": "What can I help you with today?"
          }
        ]
      },
      "finishReason": "STOP"
    }
  ]
}
```

The BYOK stream wrapper compensated by emitting the recorded stop reason
at EOF. The hosted path called the per-event mapper without that
wrapper, so completed text responses could reach consumers without a
`Stop` event.

This PR emits the stop event from the shared per-event mapper, after
processing the chunk’s content, while preserving tool-use and refusal
handling. It also removes the BYOK EOF-to-success fallback: a direct
stream ending without a completion signal now reports an unexpected-end
error instead of synthesizing successful completion.

Release Notes:
- N/A