OpenAI compatibility: add base64 embedding encoding + optional dimens…

[joonsoome]

Added

• OpenAI compatibility: base64 encoding support via encoding_format="base64" for /v1/embeddings.
• OpenAI compatibility: optional dimensions handling (truncate/pad to requested size).

Documentation

• README: Added LightRAG integration note (OpenAI embeddings + Cohere reranking tested successfully).
• README: Added Qwen Embedding similarity scaling note and recommended starting threshold COSINE_THRESHOLD=0.0 for LightRAG.
• README: Example for requesting base64-encoded embeddings and decoding back to float32.

Notes

• These updates maintain full compatibility with existing OpenAI SDK usage; default remains encoding_format="float".