markdown: Render soft breaks as line breaks in agent markdown

[tredondo]

Agents sometimes output status lists with bullets represented by emojis - see #57372. These currently get rendered on one line, as opposed to bullet items like - or *, because single newlines within a paragraph are collapsed into spaces - standard CommonMark soft-break behavior in prose contexts (e.g. README.md). But in comment/PR contexts, GitHub preserves these newlines:

✅ Did A
✅ Did B
✅ Did C

Users reasonably expect agent output to be rendered following the comment-style convention.

What this PR does

This PR adds a soft_break_as_hard_break: bool field on MarkdownStyle and enables it for MarkdownFont::Agent, so a single newline within a paragraph in agent output renders as a line break instead of a space. The field defaults to false to preserves CommonMark behavior everywhere else. In themed_with_overrides, sets it to true for MarkdownFont::Agent. The rendering switch in MarkdownElement::request_layout then emits "\n" instead of " " for MarkdownEvent::SoftBreak when the flag is set. This matches GitHub comments and Obsidian default behavior, and prevents emoji + trailing newline content from being collapsed onto a single line in agent responses.

Scoped strictly to MarkdownFont::Agent. Hover popovers, diagnostics, markdown preview, README rendering, etc. are all untouched.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior - visual tests completed:

• Performance impact has been considered and is acceptable

Closes #57372

Release Notes:

• Improved agent panel markdown rendering: single newlines within agent responses now render as line breaks, matching the convention used by GitHub comments and Obsidian.

[smitbarmase]

Thanks!