fix(cohere): omit null document when return_documents=false

[joonsoome]

Fixed

• Cohere-compatible rerank endpoints now omit the document field entirely when return_documents is false, instead of returning document: null. This avoids validation errors in strict clients (e.g. LiteLLM’s RerankResponse schema).
• Cohere v2 rerank tests tightened to assert that document is absent when return_documents=false, preventing regressions.

Changed

• Example .env.example updated to reflect the current default configuration (MLX Qwen3 embedding model, DIMENSION_STRATEGY=hidden_size, and clarified reranker settings).

Thanks for the bug report @oatmealm

#11