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:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior - visual tests completed: 
<img width="892" height="632" alt="image"
src="https://github.com/user-attachments/assets/1aacb5da-edc8-49fd-bfe9-d37a22ae2d3f"
/>

- [x] 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.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>