#23608: feat(memory-lancedb): support custom embedding baseUrl + prefixed model ids

## Summary - add optional `embedding.baseUrl` in memory-lancedb config schema + parser - pass `embedding.baseUrl` to OpenAI client as `baseURL` when provided ## Scope This PR intentionally contains only baseUrl support for embeddings. No model-id mapping, no unrelated file changes. ## Backward compatibility No behavior change when `embedding.baseUrl` is omitted.