test: add relationship traversal tests for MemoryDatabase

[mshakeg]

Description

This PR adds comprehensive test cases for entity relationship handling in the MemoryDatabase class. All test cases currently fail, highlighting that the current implementation does not properly support relationship traversal as defined in the GraphQL schema.

The test cases cover:

• One-to-Many relationships (Transaction -> TransactionReceipt[])
• Many-to-One relationships (TransactionReceipt -> Transaction)
• Interface Implementation (Transaction.sender as Owner)
• Derived Relationships (User.organizations via @derivedFrom)
• Deep Relationship Traversal (Transaction -> Receipt -> Transaction -> Receipt)

Intent

These test cases are submitted as a first step in a test-driven development approach. While I don't have the capacity to investigate and implement the fixes at the moment, I hope these tests provide clear expectations of how entity relationships should work in the MemoryDatabase.

I invite the Sentio maintainers to take over from here:

1. Investigate why these relationship traversals are failing
2. Implement the necessary changes to make these tests pass
3. Add any additional test cases they deem necessary

Related issue: #1096