test: comprehensive unit tests (160), integration tests (13) for VoyageAI and MongoDB

- Expand VoyageAI tool tests: metadata, all models, edge cases, error codes (60 tests)
- Expand VoyageAI block tests: structure, subBlocks, conditions, params edge cases (44 tests)
- Expand MongoDB utils tests: connection modes, URI building, all validators (56 tests)
- Add live integration tests: embeddings (7 models/scenarios), rerank (5 scenarios), e2e workflow
- Integration tests use undici to bypass global fetch mock
- Tests skip gracefully when VOYAGEAI_API_KEY env var is not set

Author: fzowl