The Python Together LLM and embedder ignored TOGETHER_API_BASE, unlike
their TypeScript siblings, so users behind a gateway or proxy could not
redirect Together traffic. Add together_base_url to a new TogetherConfig
(LLM side) and to BaseEmbedderConfig (embedder side, matching the existing
openai_base_url/huggingface_base_url pattern), and resolve base_url with
explicit config value > TOGETHER_API_BASE env var > hardcoded default in
both mem0/llms/together.py and mem0/embeddings/together.py. Also register
TogetherConfig in LlmFactory.provider_to_class so config dicts routed
through Memory pick it up.
Closes#6587
kartik-mem0