Profiling streamed agent responses showed `parse_markdown_with_options`
consuming ~50% of CPU cycles. The hot path is `LinkFinder` scanning
every `Text` event, including code block content, where detected URLs
are never rendered as links.

In the profiled case, the markdown had 235k+ URLs inside code blocks,
making linkify extremely costly per-parse. This PR short-circuits the
text-merging + linkify path for code block content.

Release Notes:

- N/A