test_turn_index_snapshot fails offline when GOOGLE_API_KEY is not set

Running `tests/unit/agents/flash/test_turn_index_snapshot.py` on an environment without `GOOGLE_API_KEY` fails 4 tests during `_runner` initialization:

```console
$ uv run pytest tests/unit/agents/flash/test_turn_index_snapshot.py
E pydantic_core._pydantic_core.ValidationError: 1 validation error for ChatGoogleGenerativeAI
E Value error, API key required for Gemini Developer API. Provide api_key parameter or set GOOGLE_API_KEY/GEMINI_API_KEY environment variable.
```

### Cause
`FlashRunner.__init__` instantiates `VisualStepSummarizer`, which attempts to construct `ChatGoogleGenerativeAI` without a mock. In `_runner`, `runner.summarizer` is immediately set to `None`, but `VisualStepSummarizer` is already invoked during `FlashRunner(...)` initialization.

Patching `VisualStepSummarizer` during `_runner` creation allows the suite to run offline and deterministically as intended.

Start with tests/unit/agents/flash/test_turn_index_snapshot.py and inspect the _runner fixture alongside FlashRunner.__init__. Ensure VisualStepSummarizer is patched during runner creation so ChatGoogleGenerativeAI is not constructed without credentials. Run the named pytest file without GOOGLE_API_KEY and confirm all four tests pass offline.

Written by the indexing model from the issue text.