Support endpoint_url in HuggingFaceEndpointEmbeddings

### Submission checklist - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [x] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description I would like `HuggingFaceEndpointEmbeddings` to support an optional `endpoint_url` parameter for deployed and self-hosted Hugging Face-compatible embedding services. The parameter should be mutually exclusive with `model` and `repo_id`. Both the synchronous and asynchronous inference clients should use the endpoint URL when it is provided. ### Use Case I am running a self-hosted Hugging Face Text Embeddings Inference service on a local or private network. I need `HuggingFaceEndpointEmbeddings` to send embedding requests directly to this service while preserving LangChain's synchronous and asynchronous embedding interfaces. Currently, URLs passed through `model` or `repo_id` are rejected, and `HuggingFaceEndpointEmbeddings` does not provide an `endpoint_url` option. The available workarounds require bypassing the LangChain integration or creating a custom embedding wrapper. ### Proposed Solution Add an optional `endpoint_url` field to `HuggingFaceEndpointEmbeddings`. Example usa…