[task] Add retriever deterministic tests

[AlexanderGardiner]

Checks

• I searched open and closed issues and pull requests and did not find a duplicate.

Area

Tests

Goal

Add deterministic contract tests for Retriever.retrieve() in src/anchor/retriever.py.

The retriever is a small adapter between the embedding function and the memory store, but it defines an important contract:

• embed the incoming query string
• pass the resulting embedding to the memory store
• return the memory store results unchanged

Suggested starting points

• src/anchor/retriever.py
• src/anchor/memory.py
• docs/roadmap.md
• tests/unit/test_anchor_config.py

Suggested file locations

• tests/unit/test_retriever.py

Out of scope

• Do not add Chroma-specific tests in this issue.
• Do not add end-to-end ingest/retrieve coverage here.
• Do not introduce live embedding dependencies.

Definition of done

• Add tests for the normal retrieval path using a fake embed_fn and fake memory_store.
• Verify the raw query string is passed to embed_fn.
• Verify the resulting embedding is passed to memory_store.query(..., top_k=...).
• Verify returned chunks are surfaced unchanged.
• Verify missing embed_fn raises RuntimeError.
• Keep the tests deterministic and offline.

[AlexanderGardiner]

/triaged

[kavishkartha05]

/claim

[github-actions]

@kavishkartha05 claimed this issue. Thanks for taking it on.