Write basic tests

[nwgiebink]

Write basic unit tests to ensure the RAG system continues to work as expected as we modify it. Automatically run when creating a pull request

Examples:

• RagService orchestration flow is correct -- with fake docs+model, assert deterministic outputs
• Similarity search valid: given a few docs of known content and similarity, assert similarity(A, B) > similarity(A,C)
• chunking: assert no duplicates, assert no chunks greater than max chunk size

Requires:

• pytest
• GitHub Actions and CI configuration
• configure branch protection rules