API Explorer: omit empty authentication and servers pages - #4118

An API product with no servers or no security schemes no longer gets an empty Servers or Authentication page.

Affects: API reference, Isolated builds

Prompt summary: Hide the servers page when the API spec does not declare servers, instead of showing "This API does not declare servers." Also hide the authentication page when the spec has no schemes, matching how bump.sh keeps Authentications out of an empty state and keeps Servers out of the sidebar.

Why

• Specs that omit servers still produced a /servers page whose body is "This API does not declare servers."
• Bump.sh on elastic.co/docs/api leaves that URL as an empty page and does not list Servers in Topics. We omit the page so the URL 404s instead of shipping a dead document.
• Specs that omit security schemes had the same empty Authentication page. Bump.sh lists Authentications when schemes exist. We omit the page when they do not.

What

• StructuralNavigationItem.Create adds Authentication only when HasSchemes is true, and Servers only when ReadServers is non-empty.
• The generator then writes no HTML or markdown for the omitted kinds.
• authentication and servers stay reserved child slugs even when those pages are omitted.

Notes

• Setting Hidden would still write the page. OpenApiGenerator.RenderNavigationItems walks every leaf.
• We do not copy bump.sh's empty 200 /servers URL.

Verify

dotnet test tests/Elastic.ApiExplorer.Tests/
# Create_EmptyDocument_OmitsAuthenticationAndServersPages
# Create_NoSchemes_OmitsAuthenticationPage
# Create_NoServers_OmitsServersPage
# Generate_WritesDistinctOutputTreesForMainAndReleasedMajors