All other search DTOs (SemanticSearchBodyDto, HybridSearchBodyDto,
BrainQueryDto, BrainSearchDto) already enforce @MaxLength(500) on their
query fields. SearchQueryDto.q was missed, leaving the full-text
knowledge search endpoint accepting arbitrarily long queries.
Adds @MaxLength(500) decorator and validation test coverage.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>