Function embed_openai 

async fn embed_openai(
    config: &EmbeddingConfig,
    texts: &[String],
) -> Result<Vec<Vec<f32>>>

Call the OpenAI embeddings API with retry/backoff.

Sends a batch of texts to POST https://api.openai.com/v1/embeddings and returns the embedding vectors in input order.

Retry strategy:

• HTTP 429 or 5xx → retry with exponential backoff
• HTTP 4xx (not 429) → fail immediately
• Network error → retry