Setup `testing` framework for the new modules

[Jabolol]

Build test infrastructure and test suite for the refactored factory system:

Infra:

• Mock GraphQL server for testing `graphql_factory`
• Mock REST API endpoints for testing `rest_factory`
• Mock GCS for testing chunking/state management
• Mock BigQuery for testing load jobs
• Test fixtures for common scenarios (pagination, errors, retries)

Common Modules:

• Chunking/resumability module
• Retry/error handling module
• Transform/serialization module
• Refactored graphql_factory
• Rebuilt rest_factory

[linear[bot]]

OSO-1157