Replace SERVICE wikibase:label with native SPARQL for QLever compatibility

QLever SPARQL engine doesn't support the SERVICE wikibase:label construct.
Replace it with native SPARQL using OPTIONAL/FILTER/BIND/COALESCE pattern
via the get_label_for_variable() utility function.

Changes:

• Rename get_service_wikibase_label() to get_variable_labels() in column.py
• Move label generation logic into column classes using inheritance:
  • AbstractColumn: base implementation for entity labels + helper method
  • PropertyColumn/SitelinkColumn: override to add value labels
  • TextColumn: inherits entity-only labels from AbstractColumn
• Update property_statistics.py to call column.get_variable_labels()
• Update all 208 tests to expect native SPARQL label patterns

This makes queries compatible with both Wikidata Query Service and QLever."