fix: return 409 when an embedding is used by RAG

[Absamad-dew]

Summary

• return 409 Conflict before deleting an embedding referenced by a RAG collection
• preserve the existing 204 No Content path for unreferenced embeddings
• add repository and route regression coverage for the dependency check and side effects

Rationale

RagCollection.embedding_id uses ON DELETE RESTRICT, so deleting a referenced embedding currently reaches the database and surfaces an unhandled foreign-key error. The explicit dependency check keeps that data-integrity policy, avoids poisoning the SQLAlchemy transaction with an expected constraint failure, and gives the client an actionable response. It does not translate unrelated IntegrityError exceptions into 409 responses.

Validation

• uv run --frozen ruff check giga_agent/models/rag.py giga_agent/routes/embeddings.py tests/models/test_rag_collections_repository.py tests/routes/test_embeddings_router.py
• uv run --frozen ruff format --check giga_agent/models/rag.py giga_agent/routes/embeddings.py tests/models/test_rag_collections_repository.py tests/routes/test_embeddings_router.py
• uv run --frozen pytest tests/routes/test_embeddings_router.py tests/models/test_rag_collections_repository.py (21 passed)

Closes #86