Support several GeoJSON sources in one Leaflet map

[JeroenDeDauw]

Fixes #831

geojson becomes a list parameter, so {{#display_map:geojson=A;B}} and the Leaflet
result format show several GeoJson pages or URLs on one map. Each source is fetched
separately; sources that fail are dropped without affecting the rest.

The delimiter is ; rather than ParamProcessor's default ,, since commas are common
inside GeoJson page titles and URLs, and ; is what the other Maps list parameters use.
An existing value containing a semicolon now splits into several sources, which the
release notes call out.

The visual editor replaces the whole page it was opened on, so GeoJsonSource and
GeoJsonRevisionId are non-null only when exactly one non-empty value was given and it
resolved to a page in the GeoJson namespace. The count comes from the values given rather
than from which fetches succeeded, so a source becoming reachable or unreachable can never
turn the editor on for a map that shows several. The namespace condition also fixes a
pre-existing bug: the fetcher resolves any page whose content is JSON, so
geojson=MediaWiki:Maps used to show an edit button that saved to GeoJson:Maps.

Two further failure paths are fixed here: map data still in the parser cache after an
upgrade holds a single GeoJSON object, which the editor would have opened empty over a page
that still had features; and a source whose content is a bare JSON scalar made the fetcher
throw, taking the whole page render down.

Considered and omitted: each source as a separately toggleable overlay in the layer control
(a distinct feature); de-duplicating identical sources; Google Maps, which has no geojson
parameter. Left open: a cap on the number of sources. Wikitext could already trigger as many
fetches with one map per source, but Special:Ask takes result-format parameters straight
from the request and is not parser-cached, so one anonymous request can now trigger N
server-side fetches instead of one.

_{AI-authored — Claude Code, Opus 5 (max); detailed spec from @JeroenDeDauw, no redirections; diff not yet human-reviewed; every new test seen failing first, mutation-tested, reviewed by four independent AI passes whose blocking findings are fixed here, full PHPUnit and QUnit suites plus PHPStan and phpcs run locally, rendered map and editor gating checked in a browser; CI green.}