Reject NeoWikiSubject as a page main content model

[JeroenDeDauw]

SubjectContentHandler inherited the permissive default canBeUsedOn(), which
returns true for any title. Because MediaWiki grants editcontentmodel to
logged-in users by default, a user could set an arbitrary page's main content
model to NeoWikiSubject, producing an orphan page that is not projected to the
graph and renders blank.

NeoWikiSubject is only ever valid inside the dedicated subject slot ("neo"),
whose content model is fixed by its slot role via defineRoleWithModel. That
slot role compares the model directly and never consults canBeUsedOn, so
overriding it to return false locks the model out of every main-content-model
path (the content-model change UI, the edit API, page moves, and imports)
without affecting subject slot writes.

_{AI-authored — Claude Code, Opus 4.8 (max); detailed spec from @JeroenDeDauw, no revisions; diff not yet human-reviewed; TDD (tests seen failing before the fix), full PHPUnit suite (1906) and phpcs+phpstan green locally, CI green; AI-reviewed (mutation + read-only code review), no blocking findings.}