Create layer_2_semantic collection and warn on unknown vector collect…

[saeed-taj]

Fixes #247

• VectorStore now creates the layer_2_semantic collection (Chroma name pipeline_layer_2_semantic), so the location embedding that ingestion/routes.py already pushes there is actually stored.
• store_vector() now logs a warning when the collection name isn't registered. It still returns False, so callers behave as before. A missing client still returns False quietly.

Notes

• The collection name was kept as layer_2_semantic, as discussed on the issue.
• Backfilling embeddings for existing posts is left out of this PR.

Tests

New tests/test_vector_store.py, using a temporary persist directory so it doesn't touch ./data/chroma_db:

• the layer_2_semantic collection is created
• a location vector is stored and store_vector returns True
• an unknown collection name logs a warning and returns False

[madhavzanwar]

Thanks for working on this. I fetched the branch and ran it: the 3 new tests pass, the full suite passes (352 passed, 1 skipped), and the CI flake8 gate is clean. It matches what I tried on my side.

Small things:

• In test_location_vector_is_stored the metadata key is location_test, but the route uses location_text.
• "layer_2_semantic" : has a space before the colon, and tests/test_vector_store.py has no newline at the end of the file.