Add DTO validation for FederationCapabilities to ensure proper structure.
- Create FederationCapabilitiesDto with class-validator decorators
- Validate boolean types for capability flags
- Validate string type for protocolVersion
- Update IncomingConnectionRequestDto to use validated DTO
- Add comprehensive unit tests for DTO validation
Fixes#295
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
