- Add get_file_uploader() method to BaseLLM (returns None by default)
- Implement get_file_uploader() in Anthropic, OpenAI, Gemini, Bedrock
- Pass both sync and async clients where applicable
- Update uploaders to accept optional pre-instantiated clients
- Update factory to pass through client parameters
This allows reusing authenticated LLM clients for file uploads,
avoiding redundant connections.