feat(ccip): add supportsV2Features to lanes and downstream error handling

- Add supportsV2Features boolean to TokenDirectoryLane for easier null value interpretation
- Add downstream API error pattern for verifiers (belowThreshold: null, aboveThreshold: null)
- Update LaneVerifiers type to allow null arrays for error cases
- Update CCVLaneConfig type to allow null base/threshold arrays
- Add buildVerifiersResponse helper method in token-directory service
- Update OpenAPI schema with supportsV2Features in lanes and nullable verifier arrays
- Add comprehensive documentation (docs/CCIP-API-TOKEN-DIRECTORY.md) with:
  - Recommendation section for handling null values based on supportsV2Features
  - 5 verifier scenarios with concrete curl commands and API responses
  - Code examples for TypeScript consumers
- Update tests for new lane structure

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: aelmanaa