fix: resolve visual summarizer from agent model configuration

[lihujun101]

When agent.flash.step_summarizer.model is an empty string, VisualStepSummarizer tries to resolve summarizer as a utility node. The configuration defines it as an agent node, so the lookup fails and the exception handler falls back to Google, potentially reporting a missing Gemini API key even when another provider is configured.

Remove is_utils=True so this branch reads nodes.summarizer through the existing agent model resolver. The change is one line in one file. Explicit nonempty step_summarizer.model values still follow the existing Google path; this is a narrow fix related to #95.

Validation: offline checks with a mocked model factory confirmed that the empty-model path selects the configured OpenAI model and an alternative Anthropic model without invoking the Google fallback. git diff --check passed. No live model or device calls were made.