Restore MapsFactory::getLeafletLayerDefinitions(), removed in 14.0.0

[kghbln]

Fixes #936

Maps 13.1.0 added the accessor together with the $egMapsLeafletLayerDefinitions setting (#921). The Semantic Result Formats filtered format resolves custom layer names in its map view layers parameter through it, guarded by method_exists() so Maps stays an optional dependency. 14.0.0 removed the accessor, so on 14.x the guard fails and custom layer names are silently skipped, the same degradation as Maps not being installed at all.

The restored accessor reads the effective settings, so unlike the 13.1.x one it also returns definitions set on the MediaWiki:Maps page. It mirrors LeafletService, keeping LeafletLayerDefinitions as the single normalization and hardening path, and the MediaWiki:Maps read stays lazy, at parse time.

_{AI-authored — Claude Code, Fable 5 (max); regression found and fix designed in-session, one-line asks from @kghbln, no revisions; diff reviewed by @kghbln before pushing; new regression test not run locally (no MediaWiki in the sandbox), CI is the gate; the degradation chain was verified statically against the 13.1.0-14.1.1 tags and the SRF client code.}