When tc.snippet === tc.preview (common for no-progress tools where
both are set to the same result_snippet), the detail block would show
identical content as the header. Skip the detail block in this case.
This also handles the reload-path where derived entries get snippet
populated but no preview, so displaySnippet falls back to the snippet
content for the header — same deduplication applies.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
