fix(embeddings): default to OpenAI provider when env vars are configured (#551)

[evanclan]

Summary

Fixes #551. When provider is omitted from get_provider() (the default for MCP tool calls like semantic_search_nodes), the function used to fall straight back to the local sentence-transformers provider even when CRG_OPENAI_API_KEY and CRG_OPENAI_BASE_URL were configured. This made OpenAI-compatible embeddings (e.g., Ollama, vLLM, LocalAI, LiteLLM) unreachable from AI assistants unless they explicitly passed provider="openai".

Problem

semantic_search_nodes and other callers that omit provider relied on EmbeddingStore forwarding provider=None to get_provider(). That path immediately dropped to the local default without checking whether OpenAI env vars were present, so configured cloud/local OpenAI endpoints were ignored.

Solution

• After normalizing the provider name in get_provider(), if no explicit provider was given and both CRG_OPENAI_API_KEY and CRG_OPENAI_BASE_URL are set, route to the openai branch instead of local.
• The existing openai branch already validates CRG_OPENAI_MODEL and raises a clear ValueError if it is missing, so callers get actionable feedback rather than a silent fallback.

Out of scope

• Did not change behavior when provider="local" is explicitly passed; local still wins in that case.
• Did not add defaulting for Google or MiniMax env vars; those require explicit opt-in (GOOGLE_API_KEY, MINIMAX_API_KEY) and the issue only covered OpenAI-compatible endpoints.
• Did not touch prewarm_local_embeddings() startup behavior; that is a separate concern.

Test plan

• Added test_none_defaults_to_openai_when_env_configured to verify get_provider(None) returns an OpenAIEmbeddingProvider when the OpenAI env vars are set.
• Existing test_none_and_empty_default_to_local still passes when no OpenAI env vars are configured.
• uv run pytest tests/test_embeddings.py -q — 99 passed.
• uv run --extra dev ruff check code_review_graph/embeddings.py tests/test_embeddings.py — all checks passed.
• Full CI suite on the PR.

Related

• [Feature]: semantic_search_nodes ignores CRG_OPENAI_* env vars and always falls back to local provider when provider is not explicitly passed #551 — semantic_search_nodes ignores CRG_OPENAI_* env vars when provider is not passed

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