SearchIndex: reorganize and expand test suite

Move all Search tests from _test/tests/inc/Search/ to _test/tests/Search/
to match the dokuwiki\test autoloader convention. Fix namespaces from
tests\*

SearchIndex: reorganize and expand test suite

Move all Search tests from _test/tests/inc/Search/ to _test/tests/Search/
to match the dokuwiki\test autoloader convention. Fix namespaces from
tests\* to dokuwiki\test\* so all tests work in isolation.

Extract inline test helpers into separate autoloadable mock files:
TestDirectCollection → MockDirectCollection, TestLookupCollection →
MockLookupCollection, TestFrequencyCollection → MockFrequencyCollection.

Rename AbstractIndexTest → AbstractIndexTestCase to fix PHPUnit warning
about abstract classes with Test suffix.

Replace dead xxxRealWord() with proper testWildcardSearch() verifying
exact token matches and frequencies for all three wildcard types.
Add testTokenizedPageSearch() using a dedicated test data file. Add
testNoMatchReturnsEmptyFrequencies() which exposed a bug in Term where
uninitialized $tokens/$frequencies caused crashes on zero-match terms.

Replace fulltext_query.test.php with modern QueryParserTest in the
Search\Query namespace.

Add new test files:
- LockTest: acquire/release, reference counting, stale lock override,
foreign lock rejection, releaseAll, independent locks
- NamespacePredicateTest: filter/exclude, sub-namespaces, partial prefix
safety, empty sets, score preservation
- PageSetTest: intersect, unite, subtract, isEmpty
- QueryEvaluatorTest: word lookups, AND/OR/NOT, namespace filtering,
combined queries, partial namespace prefix safety

Fix Term.php: initialize $tokens and $frequencies to [] instead of null.

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