test: tests for KnowledgeGraph::articlesInCategories()

[gesinn-it-gea]

Goal

includes/KnowledgeGraph.php:573-620 — DB query logic with a version branch (MW_VERSION >= 1.45 vs. older), currently completely untested.

Test scenarios

• Articles in a known category are returned correctly (list of Title objects).
• limit/offset are passed correctly to the DB query (pagination).
• An empty category (no members) returns an empty array.
• A category name with spaces is correctly normalized to underscores (str_replace(' ', '_', $category)).
• Both code paths (JOIN query for MW >= 1.45, simple query for older versions) are covered, as far as the CI matrix version allows.

Mocking pattern

• Integration test with MediaWikiIntegrationTestCase recommended: create real test pages with [[Category:...]] and query them via the method (DB access via wfGetDB, not meaningfully mockable).

Part of the "raise PHP coverage to ~70%" effort.