refactor: replace httpx client with NextcloudClient in upload command

- Use NextcloudClient with BasicAuth instead of raw httpx
- Replace direct HTTP POST with notes.create_note() method
- Add close() method to LLMProvider Protocol for proper cleanup
- Fix type annotations for dataset iteration

This improves code reuse and consistency with the rest of the codebase.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/rag_evaluation/llm_providers.py

@@ -26,6 +26,10 @@ class LLMProvider(Protocol):
         """
         ...
 
+    async def close(self) -> None:
+        """Close the provider and release resources."""
+        ...
+
 
 class OllamaProvider:
     """Ollama provider for local LLM inference."""