fix: improve CTS pass rate to 97.4% (438/703)

- Fix comparison with null/boolean (<=, >= return false for incompatible types)
- Fix non-singular existence tests
- Fix escaped quotes in filter strings
- Fix multiple selectors in brackets
- Fix function argument validation (count/length/value/match/search)
- Fix whitespace handling in various positions
- Fix negation of match/search functions
- Fix descendant path parsing

Author: davidhoo