Simplify the Subject move

[JeroenDeDauw]

Follows-up to #1356, into its branch. One commit per item, so any of them can be dropped on its own. User-visible behaviour is unchanged; the one API-visible difference is under the second item.

• One read of the source page. PageSubjects::without() answers a copy without the Subject. The move writes the copy, and the rollback writes back the object it read, which nothing has mutated. The second read of the source page goes, and with it the reliance on repository reads handing out distinct objects, which SubjectRepository never promised.
• One not-found outcome for the source. A source page that vanishes between the read check and its write is presented as the Subject not found: it is written first, so nothing has been written yet. presentSourcePageNotFound goes. The response is still 404, with the message Subject not found instead of Page not found.
• One error path in the REST client. moveSubject no longer reads a server message off a non-ok Response. The production client rejects on every non-2xx but 422, so that branch never carried one. The rejection path still delivers the server's message to the dialog, unchanged.
• Page creation behind the service registry. MediaWikiPageCreator wraps mw.Api().create; MoveSubjectDialog consumes its outcome and reads the Subject repository through NeoWikiServices instead of the NeoWikiExtension singleton. The dialog no longer knows MediaWiki error codes.
• Three @group Database tests dropped: source-page 403, source untouched on an unreadable target, edit summary on both pages. MoveSubjectActionTest covers each; the target-page 403 test keeps proving the Authority wiring.

Considered, omitted:

• Replacing dropFromRegistryOnceUnlisted with a plain listing reload: the moved Subject's registry copy carries its old page, so dropping it does real work.
• Sharing the Codex lookup handling between PagePicker and SubjectPicker: the largest duplication in the PR, but it means reworking SubjectPicker; a PR of its own.

_{AI-authored — Claude Code, Fable 5.1 (max); @JeroenDeDauw asked for a follow-up PR that reduces complexity after an in-session assessment of the original, scope chosen by the assistant, no redirects; diff not yet human-reviewed; full PHPUnit suite and tsci green locally on the branch, each new test mutation-checked, a move onto a page that did not exist yet exercised in a dev wiki.}

Production notes

Design, diff review and this text by Fable 5.1 (max); implementation and tests by an Opus 5 (max) subagent from a written spec. The dev-wiki check covered the page-creation path only; the other paths rest on the retained unit and Database-group tests.

[JeroenDeDauw]

Ugh. Try to get claude to remove lines. 🤔