preview_snapshot always captured a screenshot, even when the agent asked
for neither includeImage nor save, which is the default. A desktop tab the
Browser panel is not showing draws no frames, so Page.captureScreenshot
never answers and the whole snapshot failed after 10 seconds, while the
text reads it waited on had finished long before.

The tool now asks the host for a screenshot only when it will show or save
one. Without it, the snapshot skips the capture and the screencast pause
around it, and the result has no screenshot field.

Refs #16567

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>