Skip OpenAI model registration when no API key is configured

[antisilent]

Closes #1445.

Summary

• register_models and register_embedding_models in the built-in OpenAI plugin now return early if no API key is found (via llm keys, keys.json, or OPENAI_API_KEY). If credentials are not configured, the plugin is invisible.
• Test fixture (env_setup) now sets a dummy OPENAI_API_KEY so existing tests continue to work, and scrubs LLM_MODEL, LLM_EMBEDDING_MODEL, and LLM_EMBEDDINGS_DB to prevent host environment variables from leaking into tests.
• Added test_openai_models_not_registered_without_key to cover the new behavior.

Test plan

• uv run pytest passes (755 tests)
• llm models shows no OpenAI models when OPENAI_API_KEY is unset and no key is stored
• llm models shows OpenAI models normally once a key is configured