Fix broken XHTML in Atom feed for notes with line breaks

What does this MR do and why?

Notes in the project activity Atom feed were rendered using markdown_field, which uses the default Banzai pipeline and produces standard HTML with bare <br> tags. The <summary type="xhtml"> element in the Atom feed requires valid XHTML, where <br> must be self-closing (<br />).

This fix adds the missing Events::RenderService call with atom_request: true in ProjectsController#show for the atom format, consistent with GroupsController, UsersController, and Dashboard::ProjectsController. This pre-renders and caches the events using the correct XHTML-aware pipeline, so that markdown_field in the view produces self-closing <br /> tags.

Note: the _event_note.atom.haml view continues to use markdown_field(note, :note) (rather than markdown(note.note, pipeline: :atom, ...)), which allows it to use the pre-rendered cache from Events::RenderService and avoids per-note re-rendering with all its associated lookups.

References

Fixes #361722

Screenshots or screen recordings

Before	After
<summary type="html"><p data-sourcepos="1:1-1:10" dir="auto">foo<br>bar</p></summary>	<summary type="xhtml"><div xmlns="http://www.w3.org/1999/xhtml"><p dir="auto">foo<br />bar</p></div></summary>

How to set up and validate locally

1. Post a comment with a line break on any issue
2. Visit the project activity Atom feed at http://localhost:3000/group/project.atom?private_token=TOKEN
3. Before fix: feed contains bare <br> tags, producing invalid XML
4. After fix: feed contains self-closing <br />, producing valid XHTML

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